Hiya Fossala...
I guess £350 sounds a fair price if the bike is 'as new'. But thats an easy description for a seller to give, does the condition match the description ?
I'd be looking carefully at the transmission, looking for wear and tear.
I privately sold a 4 year old 9 speed Bianchi Via Nirone for £250...its condition was fair, with the usual wear to the transmission. Part of me felt i didnt get enough, part of me felt its only worth what someone was prepared to pay...try and work him down on price a bit perhaps.
Give the bike a good once over, preferably ride it, make sure the gears work ok, adjust the seatpost height, make sure its not siezed, if it goes well, looks ok, you're happy its in good condition,...find a price you're both happy with...and buy it.