It's been happening to other sellers.
As far as I know, there's no way to prevent eBay from making these changes, but since they aren't terms to which you, the seller, agree (as you did not indicate that you were willing to consider offers, nor did you list with auto-accept for half the listed price), you'd have every right to refuse to complete any sale made under the conditions of eBay's tampering with your listing.
I half-jokingly suggested a disclaimer for listings, but you'd have to be very careful about how to word it so as not to render yourself a target by eBay. There's also the issue that if you cancel a sale, you have only three choices for doing so - and unless you choose "buyer requested cancellation" or "there's a problem with buyer's address", you'd end up with a defect. I suspect you'd have to get onto the phone with eBay every single time and point out that you didn't list the item with best offer enabled, and you're not willing to sell at a price that you did not okay.
eBay cannot actually force any seller legally to sell under those conditions, even if eBay CS reps quote eBay's User Agreement - because it's not possible for someone to agree to a condition of contract which negates their rights under Australian consumer law. It is up to the seller to set the selling price.
However, bear in mind that eBay can close down any seller's account - for any reason, or even without a reason, and certainly without recourse. That's why it is best to find ways to solve the issue without being combative.
Here are some other threads about this very issue:
https://community.ebay.com.au/t5/Selling/Automatic-Best-Offer-or-Ebay-finds-new-ways-to-annoy-me/m-p...
https://community.ebay.com.au/t5/Selling/Make-an-offer-added-to-new-listings/m-p/2182953
https://community.ebay.com.au/t5/Selling/quot-We-ve-allowed-buyers-to-make-offers-to-help-you-sell-q...
https://community.ebay.com.au/t5/Selling/Occasional-sellers-penalised/m-p/2144539#M168565
I would suggest that you keep a record of every time you have remove Best Offer options with auto-accept at 50% of your listed price. Time and date and item number should be recorded.
Then contact eBay via the Have Us Call You option, and record the call with an external recording device; get the CS rep's agreement to this at the start, of course. Keep notes of the conversation - name of rep, time and date, substance of call, and finally a summary.
The call should draw the CS rep's attention to your tampered-with listings, and state that you hereby want to politely inform eBay that you don't give eBay permission to change your sales terms, and that you have listed the minimum price (BIN price) that you can accept. Point out how many times the listings have been tampered with without your permission. Politely mention that Australian Consumer Law exlicitly states that businesses are entitled to set their prices as they wish; ask that your listings not be tampered with. (You may want to talk about being obliged to cancel transactions if someone makes an offer on your item for a price less than your stated BIN price, but be careful in saying anything along those lines. It's probably best not to mention that recourse - cancelling sales - as you may simply red-flag your account. If it comes to your needing to cancel sales in such conditions, you could think about the situation then rather than borrow trouble by telegraphing your possible/probable actions well before there's any need to do so.)
Get the CS rep to agree that your listings will not have their prices edited without your knowledge and against your will.
If the CS rep cannot agree to that, make sure you explicitly summarise this and get the rep's agreement that this is what eBay refuse to do. For example,
You: "So, just to clarify, you are saying that eBay will continue to tamper with my listings' prices and will enable Best Offer even though I have explicitly disabled Best Offer?"
CS rep: "That's right. Blah blah blah."
You: "And - just to continue to clarify - you are saying that eBay will continue to change my listings from having a set price, and enable an auto-accept for an offer that's more than half my listed price, is that correct?"
CS rep: "Yes, blah blah blah."
You: "And you agree that I am specifically instructing eBay now not to change my prices without my knowledge or agreement?"
CS rep: "Yes, I understand that you blah blah blah."
You. "And you are saying that even though I have specifically instructed eBay not to change my prices in this way, eBay will in fact ignore my instructions and continue to tamper with my listings against my will?"
Cs rep: "Yes, blah blah" [probably something about User Agreement]
You need to break down exactly what has been said. Remember to stay calm and focused; the summary isn't a time to harangue the CS rep, but rather to make sure each party understands what has been said and/or agreed upon.
And if the rep promises the tampering won't happen again, you need to have that in writing, recorded, and/or evidenced by your detailed notes and phone records. Why? Because if the tampering reoccurs, you need to have evidence that it was agreed that it would not occur.
If it comes down to taking some sort of action or lodging a complaint, you'll be in a better position if you have records supporting what you assert has occurred. Remember, though, that the best remedy is polite communication with eBay. It's unknown at this stage if there's anything you can do through communicating with eBay to stop this sort of interference, but it is certainly the least disruptive and bothersome avenue, so give it your best try.
