I can sometimes figure out a more succinct and / or effective way to explain it, several hours later... sometimes my attempts to clarify makes things worse

But if it helps, there is no checkout process attached to the make an offer feature. If a buyer sends you an offer, and you accept it, there's no way for eBay to force the buyer to checkout at that point, but the sale itself is still made.

If you have best offer on an item but the buyer still just uses Buy It Now at full price, immediate payment can be forced.