Nothing but the obvious: it may show signs of wear, and some of the battery's finite life will be used up. It won't come with any kind of warranty or return policy, and there's also no knowing how the prior owner may have mistreated it in ways that don't show. It's a gamble; you have to decide if the price cut warrants it.
In my experience though, the Touch is a fairly robust, solid-state device. If it doesn't look abused, it's probably ok.