How does Vendor inventory work?

[clouddynasty]

So I've gotten the concept of 'fishing' a vendor for items by buying off junk and moving it to a new vendor. However, it is not enough to buy an item unless it clears that item off the list, or so it seems? I.e. if vendor has 4 short beers buying 1 and then closing/opening will not move around his stock, yes?

Let's say I'm looking for giant rat ears. I will sell one of my giant rat ears as a 'marker' so that after fishing through the vendor if I come across '1' giant rat ear I will know it is mine and he had no others. Or is there a better way to do this? I'm skeptical if this even works because there have been times that I have bought off all 6 of say... 'cloth caps' and later on more will appear.

Is there a trick to fishing for a specific item that you want? Meats, components, etc. ?