Make sure you buy a legitimate retail copy. If there are pictures of the DVD, it should be black DVD(s) with the stylized X logo, and not one or more grey DVD. The grey ones are system install/restore discs and they do not work on any Macs except the model / generation with which they were sold (and they're not licensed for any other use either). So both legally and pragmatically, the grey discs will not be useful to you. If the sale shows a box then just of course make sure it's a retail Leopard box.
And all the other warnings about eBay apply. But on the fundamentals of using Leopard, yeah, sure, why not? 🙂