Don't choose an auction site simply because it's free. Look for one that attracts a decent number of sellers and bidders. Some sites keep a running total of the number of products on offer. On the day we checked eBay, for instance, it counted more than 147,000 computer products up for grabs, and more than 3,000 PCs. Add in the extras.
Most auction sites charge fees to list items.
Make sure to factor these in before setting a minimum price. You can also set a reserve price, or a minimum price below which you're not willing to sell. If the highest bid is too low, you may be better off donating the system to charity. Auction items have a better chance of selling if bidders can see what they're getting. Along with a picture or two, include a detailed description of the system, flaws and all.