I usually take my old machines to that High Street pawn shop. They pay about 25 per cent of new price.
Back in the bad old days of hard drive I used to put the machine back to showroom condition by deleting My Documents and reloading the Windows dvd. If you're worried do a total format. Modern machines have factory reset which clears all data.
There are fancy ways of recovering data from an erased hard drive but it is unlikely that your data would be sufficiently valuable to justify the cost. Three cycles of total format would defeat even Cheltenham's finest.