RIAA claims it is illegal to sell an MP3 player with copyrighted songs. I presume they say that buying such an MP3 player is also illegal.

More from the article on FMQB:

"Selling an iPod preloaded with music is no different than selling a DVD onto which you have burned your entire music collection," the RIAA said in a statement to MTV.com. "Either act is a clear violation of U.S. copyright law. The RIAA is monitoring this means of infringement. In short: seller beware." Many people have been selling their used iPods online with thousands of songs preloaded on them.

Andrew Bridges, a lawyer for eBay that specializes in copyright and trademark law, told MTV.com, "It really depends on individual circumstances. I'm not sure the law is settled. If I'm a college student and I want to supplement my income by buying 100 iPods and selling them at a significant premium, that's probably not going to fly. But if I've had my iPod Shuffle for two years and I'm tired of it and I go out and buy a 60 gig video iPod and want to sell my old Shuffle, but don't want to purge the music first, that's probably legal."