I would possibly look towards a different strategy.
Rather than deleting a page once a product is sold I would leave the page live for a while and put a sold sign on it and offer a selection of similar properties to view. After say 3 months then 301 the page back to a category page higher in the hierarchy. Or don't 301 it at all, perhaps Noindex the page after a given time frame?
I'd much rather present my visitors with a Sold sign and a list of similar properties than potentially lose them to a 404.
Don't think search engine, think visitor
Hope that helps
Aran