Given that this is the most active UK forum, especially for sales, keeping old threads around for reference is very relevant.
Not forgetting that it's also the only place where members are allowed, even encouraged, to discuss the prices of items being sold here, or other websites.
Those two aspects alone are a virtual mine of useful information for existing & new members alike.
The only real issue with old sales threads is when sellers don't update their status, such as when items are sold.