I hardly ever use Ebay, or B&S, or anything like that. The only time I buy anything used is if it is something that I just cannot find anywhere else. This was the case even when I was a completely broke ass student.
I just offer what I am willing to pay. There is pretty much nothing I absolutely need, so if I see something, I will offer a price that I am willing to pay. Because of the budgets I've set, I rarely have more than $25 to move. If it's accepted, cool. If not, I understand. When you use the "best offer", you are essentially saying that you are willing to negotiate, and nearly any decent negotiator will take that and run with it. If they are good, they will realize that if they are willing to walk away, they will not be stuck with something they didn't particularly want in the first place.