sorry to hear about your bad experience. my bike shop had given me the wrong size as well - they gave me a 20" bike and it didnt feel right ever. i kept the bike for about 300 miles before selling it. i tried out a 22.5" 7.3 and just sitting on the bike felt so much more comfortable! so i sold that first bike (09 fx 7.2) and got my '10 fx 7.7.
your current bike looks clean so i think you can definitely get $400 for it. put it on craigslist and maybe some local (free) papers in your area for maximum exposure. put it on ebay as well and keep an eye on what other 09 7.3's are going for too.
lastly, talk to that bike shop and let them know you're not doing business with them again. you should be able to try out all the bikes you have on your mind before making a decision as opposed to simply picking one out of their current inventory...