While it's not a huge issue right now, this is just a good idea in general. As Kesh said, offers that have been up for weeks and haven't sold aren't likely to get sold anyway. Thus they will accumulate forever, making the database harder and harder to browse, practically and technically. I imagine that people on low bandwidth connections are already struggling to load the entire list in a reasonable timeframe, and the coding of the market makes the browser react slowly in the meantime.
Simple solution: Make all buy/sell offers expire on their own after 7 days. Cleans up the market considerably, with minimal negative effect.
