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EDIT - sorry stawks, just read your post, but great minds think alike.....
Everard, if the items are collectable and buyers are taking the lot soon after listing, it would appear your prices are way too cheap. A collector does not usually need 20 of an item, so chances are they are buying and reselling the item for a much higher price. It is your bulk buyers who are reaping the benefit of your items, not you.
You would be much better off experimenting with your prices a bit. Raise them, quite substantially and then lower them untill the item starts to sell. That price is what the item is really worth. You can then work with more subtle price changes to either restrict sales for items of limited quantity and slightly lower prices for bulk quantity items you want to keep moving.
Charging the highest price the market will bear within reason is the simplest and most effective way to generate maximum profits from your ebay business.
ETA Sorry - mobile ghost post that happened when I pressed back on the browser.
Will add something of (hopefully) a little more substance after my coffee. 
@everard6920 wrote:
I am sorry if I came across as complaining or bragging to anyone. It was not my intention at all. I've had messages from a few customers upset at missing out on these items, and just wanted to see if there was a solution. Honestly.
The best solution would be to buy more (and more). 
Seriously, though, I understand where you're coming from. My items aren't exactly collectible, but many of them are niche and I feel very mixed emotions about being bought out of a product when it does happen, especially if it's a new product and they are the first (or one of the first) to purchase it at all, (i.e. does that mean it's a hot product that's going to sell like hot cakes and I should invest every spare cent I have on restocking, or does it mean that one person has been looking for that item for ages and bought a lifetime's supply while they could and I'll sell one a year once I restock - I've experienced both outcomes more than once).
Popular products can sell out unexpectedly, too, i.e. I have a product most people buy 1 or 2 of, but it sells almost daily. My stock purchases usually more than keep up with it, then once in a blue moon someone will buy everything I have, almost always right after I've made a large purchase from my supplier and didn't include that item because I had heaps... So I have to wait up to a month to be able to purchase new stock, and field "when are you getting X item back in stock..." questions. I made one customer very happy, but i have a lot of regulars and I feel like I'm letting the rest of them down.
Some of the stuff it happens to is handmade. The little shout of joy I get when I spent 3-4 hours of an evening assembling something and they're all gone straight away when they usually last a few weeks, is often drowned out by the groan of knowing I now have to spend another night on the assembly line, lol.
What I try to do with stock now is buy as much as I can to last a specified amount of time, covering one of two bulk purchases if possible, but actually list about 80% as availalable. When the last one is bought, I'll leave it as out of stock for a few days, then make about 90% of the remainder available, with the last few there for when that sells out and people contact me to ask when I'll be getting more (people usually do this when they need an item and relatively quickly, so basically I keep 'hidden' reserves in order to be able to fill that market).
