Presumably the buyer has yet to take possession of it.
Easiest way would be to run a complete backup of its hdd to an external USB drive. You don't mention which version of Windows is installed, but backing up would be under the control panel/Backup and restore.
If it's just files you've put on there, they should be in C:\Users. All data on other partitions should be copied across too. If in doubt, back it up.
Once done, it would be wise to perform a factory reset using the recovery partition or DVD if there is one. Ensure no personal / financial / sensitive info is left.
Without knowing anything about the sale, particularly on what was agreed between you and the buyer, that's about all the advice I can offer.
Whatever happens, after it leaves your possession, assume you will no longer be able to access what's on it.