I don't have a problem with anyone registering then immediately trying to sell something, but then again it's not my website. However I'm sure that a regular poster, someone who's shown some enthusiasm for the V-strom, is going to get more responses from any ad placed.
I think around £4000 is pretty spot on for this age of bike, but you may end up taking a couple of hundred quid less, especially if you're in a hurry. Personally I would have taken the earlier offer of £3900, since it cleary wasn't a silly offer you sometime get when trying to sell privately. If the bike is in good nic then don't take less than you're really wanting unless you're in a real hurry. And don't forget you can also get stung when selling privately, so be careful.
By the way Wisebuyer which is usually reasonably accurate has a 2010 GT at:
Retail: £4,900
Private Good: £4,030
Private Average: £3,620
Private Poor: £3,360
Part Exchange: £3,880
Trade: £3,697