Ads are not deleted immediately, because we want people to have the chance to make sure they have got what they paid for, and can use the ad as reference. Once the ads are deleted, there is no retrieving them, they're gone forever.
I'm not saying delete them immediately, but move them to the Buy/Sell/Trade Archive as soon as they are listed as sold. After a week or so there, then they can be deleted.

Sold items clear out automatically, seven days after sale.
Might want to check the settings, there's a SOLD razor from December.