It's also worth checking Craigslist.
Generally, the modern systems (whether they're licensed or not, some of the Genesis clones are licensed) fall into two categories: imperfect clones, or mediocre emulators. Some of the imperfect clones have their own charm (the Atari Flashback 2 and 2+, for instance, were designed to be hackable to run 2600 cartridges), but you're usually better off buying the real thing.