Refer to the following thread for a bit of info on what might be the problem...and possible fixes:
http://www.airsoftallies.com/aeg-discussions-f11/gg-m4-selector-switch-t4307.htmAs for the selling, you could MAYBE get $200 or so for it. Bought for about $250-$300, no?
Sounds like you've had it for a while...and any damage/problem (such as the selector switch) will drop the price dramatically...even if the problem only requires a cheap fix. Its simple...no one wants to buy something that, in their mind, is "broken".
So I'd start at the highest possible asking price...and keep your options open for bartering.
Places to sell:
-Craigslist (FREE!)
-Here (FREE!)
-Other airsoft forums (FREE)
-Or contact a local team with a website to see if they can post an "ad" there.
If you need any further help, just say the word.
You'll get that gun sold.