Sorry to hear about your loss... It is ultimately your own fault though for (a) not doing your "homework" and just trusting what the seller told you, and (b) for buying an underpowered and grossly overpriced "iCrap" device. That doesnt mean that the guy that sold you that thing wasnt a dick though...he obviously knew what he was selling you, and knew that it wasnt worth what he was asking or else he wouldnt have gotten all pissed off at you when you questioned him about a refund. There is a sucker born every minute is the old saying, that and buyer beware. Sorry bro. (You could call the local police department and see what they say, since the seller outright lied to you to sell you the device...but they may tell you to take it to small claims court, which may be your only option.)
You could have probably built a comparable PC for half the price of what you paid (if not even less)...although it wouldnt have been a Mac because Apple locks their hardware/software down, so you cant save money and build yourself one (which is BS IMHO)...or upgrade it after you buy it - you know, the normal things you would do with a PC. But its Apple and they must lock things down to maximize profits at the behest of users...gold-diggers.
Sorry to have to say this, but iMacs are pretty-much worthless to literate computer users (the only people I see using Macs are students who dont know any better, and beatnicks who think they are god's gift to man-kind). Sure, someone will probably argue with me on that...but it is true for the most part. There is no reason to own a Mac unless you like paying more for less computer...and that is beside the fact that the majority of software that is developed is for the PC platform because that is where the vast majority of the user-base is (hence the reason people have to use bootcamp to run Windows to play games or run software they want, or port games themselves to MacOS because publishers dont care to). Basically, hardcore computer users tend to prefer PC because they can build & upgrade their own rig and not have to settle for an underpowered and overpriced device that wont do what they want.
Apple is riding the success of their iPhone/iPad and using that to help push their computer sales, and to the computer illiterate consumer, the sad part is that they probably think they are getting a great deal on a shiny new device...but all they are getting is the shinyness with little to no substance IMHO.
Good luck selling it, hopefully you can find another computer illiterate person to sell it to...or someone who will take pity on you and take that device off your hands.