Here's how I see it.
DS Launched in November, 4 months before PSP came out (and at the time, a release date was only rumored, not concrete). Now there are 3 types of people who picked up a DS
#1 To play DS games
#2 To play DS games as well as GBA (GBA looks AWESOME)
#3 Something to hold them over till PSP.
Now, why so we see so many used DS's? Well, it has been four months. Some people lose intrest, some people need cash. Some think the DS is crap (Everyone is entitled to their own opinion), some people can't 'wait' till the 'good' games come out (Mario Kart, Animal Corssing, Pac-Pix, Sonic, Castlevania, FF3...).
But WHY are we seeing so many used DS'? Its simple. PSP. Well duh. But again, why are we seeing SO many, more so then what some would call 'normal'? Again, PSP, and mainly its price. No handheld in the past 10 years besides the Nomad (which had the abaility to play 800+ games right away) cost more then $200 (exception N-Gage, but its also a phone).The PSP being $250 causes alot of people to go "I want the PSP! But I can't afford $250! I do have my DS and Mario...and nothing good has come out, might as well trade it in!". And thats exactly whatis happening. If the PSP was $150, I don't think we would see as many DS' getting traded in as we do now. Most people need to get rid of their DS to fiance there PSP. But here's the kicker. Alot of those people in the next few months will be hurting when there is a flood of AAA software (Mentioned above) and will once again, rebuy there DS.
Sorry if I rambled a bit, but I honestly think we are seeing so many used DS systems is because they need something to fiance the price of the PSP and the games (Which are $40 at the cheapest).