I can really only answer this from the perspective of a seller - what I do know is that ebay's system is set up to punish sellers who receive an order for an item they can't supply (and this can happen for a variety of reasons, sometimes it's 100% the seller's fault, sometimes it's not), but this in turn leads some sellers to act in a manner where protecting themselves from eBay becomes the priority over advising the buyer and risking an immediate defect.To exemplify what I mean, if a seller contacts a buyer and cancels a transaction because they are out of stock, they get a defect that can affect their ability to sell. It also gives a bad impression to the buyer about the seller. If they mark the item as shipped, then wait for the buyer to open an INR request, they'll issue a refund, the buyer feels good about the seller because they don't blame them for a lost package and got a refund, and eBay doesn't give the seller a defect.
It's a bit different with missing items, of course, but some might be thinking if they keep up the pretense of it being purely unintentional, it will avoid more problems.
I am not condoning these kinds of actions, by the way, I think if sellers make genuine mistakes it can handled in a way that does not cause more trouble and delay for the buyer, or put the seller at risk either.
If sellers are intentionally leaving items they can supply out of the package, though, for whatever reason, I think the request route is the best way to go. You can always send a quick message or inquiry, gauge their response, and if you're still wary or don't get a satisfactory reply, it's reasonable to then advise them you feel this is the best way to move forward. (BTW, items missing from an order come under 'Not as Described', not 'Not received' - not received disputes delivery only, not as described disputes contents).
I don't know if this will help at all in terms of how to approach sellers, but I frequently have multiple-item orders, and occasionally either make a mistake and an item is not included (unintentionally), or find that I can't supply what the buyer has ordered. (And no, I would never suggest it's normal or that it can be expected, at least not from a seller who is trying to do the right thing - I am always very embarassed when it does, and grateful to the few regular buyers who are very patient and understanding with me when it's happened more than once 
).
In the first instance, if I have left out an item and the buyer actually wants the item, it is better for them to contact me directly via a message because then I can check my records, verify that it was left out, and send a replacement. If they open a request, the only option becomes a refund. I don't particularly like requests, but I accept they are a part of eBay and will not usually hold it against a buyer if they open one, particularly when the reasons are genuine. In some cases I have advised buyers to open requests as the simplest solution for us both.
If I discover that I can't supply an item (this usally happens because I've miscounted stock levels, or find a flaw that was previously undetected), then how I handle it depends on whether there are other items in the order. If it's just the one item, I contact and explain, and provide a few alternative options, including choosing a different item up to 20% more in value, or a refund. If there are mutliple items, I supply what I can of the item quantity (usually I am able to include a partial amount of what they ordered of the item I can't supply in full), but I will also issue a full refund and explain why (eg I selll things in packs, so say they ordered 50pcs and I only have 25, I'll send 25 and refund for the full 50).
I mention this because it's the kind of response I'd appreciate if I were the buyer in these kinds of circumstances, and in fact it's what I've experienced from other (non-eBay) sellers in similar circumstances, so I think it can provide a little insight into the way sellers respond when they have different priorities than their own interests.
You said you gave the neutrals a lot of thought, which I respect, and I think anyone who blames you for losing their job (if that even happened) over such neutrals is again not looking at the bigger picture, nor what actions caused you to be dissatisfied with the transactions in the first place. If there's a fly in a meal and the diner complains, you don't blame the diner for complaining, you blame the staff responsible for sending out a meal with a fly in it.
@imperatrixi wrote:
You sound like a great seller to do business with!
With your advice about which category to open a refund request, I'm not sure that it should be "item not as described" if the problem with a transaction is that an individual item purchased that has been marked sent and was tracked as delivered then is missing from the tracked parcel. To me that does seem like a case for "item not received". I guess that "delivered" is a description, is that your reasoning? Either way, if it is a tracked item and is recorded as delivered, Ebay removes the option for you to open a case for this reason "item not received" and you have to contact them for further assistance.
I guess I'm ok. 
I'm sure I've had some buyers who would not think so, but for the most part I just try to do what is right. Other sellers can be motivated to do what has the least (immediate) consequence to their business - as a buyer I would be quite angry about such practices, as a seller I still think it's wrong, but can understand the thought process behind it, and that it's not purposefully malicious in many cases, but certainly a solution that is much less than ideal for buyers. (This obviously doesn't apply to sellers who do inentionally leave items out of orders, or refuse to send an item if the price acheived wasn't high enough, but Hanlon's razor can still apply - Never attribute to malice that which is adequately explained by stupidity. In some cases it will be a scamming seller, so you can certainly attribute that to malice, but other times you've just got an imperfect person who handles things imperfectly and which impacts others negatively).
With regards to the reason selected for disputes - I understand that when you don't receive something, an 'INR' dispute is the most intuitve reason to select as the reason for the dispute, but I said what I did in my previous post because that is not how eBay's (and PayPal's) system is set up.
Item not received disputes quite literally work only on establishing postage (in the case of PayPal) or delivery (in the case of eBay). It doesn't dispute the contents of a package in any way whatsoever, so in a regular case if the buyer selects INR for a package where the seller can establish postage / delivery by PayPal's / eBay's rules, the seller will win pretty much 100% of the time.
They have specific options under not as described for packages that are missing contents. You sometimes have to jump through a few extra hoops to establish that the package didn't contain all items, but the seller can't simply turn around with just the tracking number to prove delivery of the items in question (this is one of the reasons I photograph the contents of all packages I send out, it enables me to immediately verify or disprove claims of items missing, the latter of which I have to do regularly, as well, unfortunately).
