I
HATE "make me an offer"
As a seller, I know what I want to get from a game. If someone wants to haggle, I'm good with that, but I know what I want to get. NO ONE goes into a sale not having a clue what they are looking to get for something. That is madness.
As a buyer, "make me an offer" scares the hell out of me. How do I know what you are expecting to get for something? I can offer what I think is fair, but if you think it is too low the sale is pretty much going to be void. If I offer you too much, then you got me and I could have paid less and you still would have been happy. Either way, I the buyer, am screwed.
As a seller, I price everything I am looking to sell and am open to haggle, although this rarely happens as I price everything to sell.
As a buyer confronted with "make an offer" I do one of two things: take my business elsewhere (most cases unless it is something rare or I really really want it), or I make the lowest possible reasonable offer, and maybe sometimes a little less than that. If I don't get it, then I wasn't going to anyway, and if I do I get it as cheap as possible.
As for determining exactly what to offer, I say offer whatever you think it is worth paying. Simple as that. For instance, the community center next door to us had a yard sale and was selling an old, beat up
Ms. Pac-man stand-up arcade machine. It was rough. I offered $75 and wheeled it home 5 mins. later. That was what I felt it was worth and that is what I paid. Likewise, at a recent convention I saw a boxed copy of
Pirates! (NES) and was asked to make an offer. I said $20, they said $35, I kindly declined and walked off. $20 was all I felt it was worth.
Eventually, after being around, you sort of develop an idea of what you are willing to pay for things based on how much you want them and how often you see them. From there you generate you "offers" and you stick to them.
Bon chance!