I've sold things both ways...with "OBO" listings and with "Firm" listings. I vacillate between the two, and I've yet to decide which is best. I can say with certainty that nearly every time list an item as "Firm", buyers respond with lower offers, or they want shipping included, or they want the Paypal fees absorbed. It's just the nature of bargaining.
I have sold several "OBO" listed items at full asking price with shipping and fees on top, but these are usually high-demand items for which there are many previous sales on which to base a fair asking price.
As is always the case, the market is determined by supply and demand. And, the more clearly defined the market, the better one can determine a fair asking price. Also, in my opinion, the word "Fair" is too subjective. One person's "Fair" is another person's "Overpriced". If I list an item at a fixed price, I always price it at the amount at which I believe it will sell, not at a price I hope it will sell. There is a huge difference between the two.
I have sold several "OBO" listed items at full asking price with shipping and fees on top, but these are usually high-demand items for which there are many previous sales on which to base a fair asking price.
As is always the case, the market is determined by supply and demand. And, the more clearly defined the market, the better one can determine a fair asking price. Also, in my opinion, the word "Fair" is too subjective. One person's "Fair" is another person's "Overpriced". If I list an item at a fixed price, I always price it at the amount at which I believe it will sell, not at a price I hope it will sell. There is a huge difference between the two.

