The big issue with these companies is that they say on the web it's worth XXX dollars/pounds/euros, but when they get their hands on it they have to evaluate it, and then they will often cut the offer in half, saying it's not in good condition or has issues or doesn't work. Better to go to e-bay.
There are a lot of shady places doing stuff like this (they need to make money to stay afloat). Do a google search on the company and see what other previous-users have posted.
you'll get more selling it yourself than giving it to the these guys. they will obviously offer you a lower than actual market price so they can make their profit when they sell it on.
don't really see the point to these companies. I can understand it's a cash quick route but you're losing out so why do it?