well that "potential buyer" is not very smart then if hes real. why would someone risk buying a used imac for 1400 lol, maybe a top of the line i7 maxed out but not the base imac 27 incher
selling a used imac whether or ebay or craigslist is really tough, i sold an imac before so i know, its really tough to sell it as everyone is scared it has some hidden defects and what not
Ebay is very easy to sell them on. I have sold a 15" MBP, a 13" MBP, a 27" Imac, a 24" Imac, 2 Ipad's on it without a problem, and always going for within a few hundred of my original purchase price.
Don't let this extremely uninformed individual deter you from upgrading.