Given that the previous post was closed because a few people chose to go - and stay - off-topic, let me point out a simple fact: this forum already uses one means to indicate activity in a post. Pick a sub-section of the Forum; notice those posts in RED? They indicate a lot of activity - they're hot topics!
So, why does it seem so difficult for some to have something similar when an item has sold? Color likely isn't a good choice, as there's no color that I'm aware of that makes one think of the sale of an item. Greying out of the post title is, IMHO, probably the best option, though there may be others.
Can you, dear reader, think of a way for the OP to indicate when an item has sold at the post title level (e.g., what one sees when they click on the "For Sale/Wanted" link)? Greying out the post title has already been suggested, so please think of other means.
BTW, I agree with those in the previous post that old posts should remain; deleting is not a good option. Keeping the old posts helps to build an archive of prices and comments regarding sold items.