Try craigslist, you wont get more that 40 dollars for it. The only person that I could see that would want it is some one who had a front mount and is putting it back to stock to sell it.
You must be thinking he is selling a stock WRX topmount, but i think he is selling a stock STI topmount. this is a popular mod for many wrx people and you can get decent money for it. ive seen ones on nasioc or ebay go for 150-250$ depending on condition
Yep, that piece is relatively high in demand. It's a low-budget, under-hood, functional piece of bling for us WRXers. If it's in really good condition, I'd just list it for $300 o.b.o. and you'll probably be offered $2-something.