Used DS games are more trustworthy than other disc-based games. They are less prone to damage and tampering, and previous game data can be easily erased.
Of course, why wouldn't you? As long as you are buying them through a registered and reputable company they should be fine - if they are not you can always send them back for a refund.
It all depends on who is selling the game. DS cartages are usally pretty safe as long as they do not look damaged from the outside. But, just to be safe, if it looks sketchy, do not buy.
