I was just talking about the PSP with my kids yesterday. They now both have one each and one just plays Yu-Gi-Oh Tag Force games on his, while the other just plays Sonic games on his. Their DS's never get put down by comparison.
Definitely Trade! Not sure what you'll get if you trade via a shop - you'd be better off selling it via Amazon or eBay - that way you should get about £120 or so - enough for a DS Lite and a game or two.
There's loads of brilliant games available on the DS. Just browse the bestsellers at Play.om or a similar site!
http://www.play.com/Games/DS/6-/TopSellers.html