I needed an iPad 1 to test my next release, and instead of using eBay as I usually do, I noticed that Amazon has a substantial selection of used items. I ended up getting an iPad 1 in great condition (perfect screen, one scratch on the back) for $125, without the extra hassle of eBay. If you need an older device, either for compatibility with the older 30-pin dock connector or just to save money, give it a try. Here are links for iPad models 1, 2 and 3:
Used iPad 1 - limited to iOS 5, which excludes some app features (http://www.amazon.com/gp/offer-listing/B002C7481G/sr=/qid=/?ie=UTF8&camp=1789&colid=&coliid=&condition=used&creative=390957&linkCode=ur2&me=&qid=&seller=&sr=&tag=setlist-20)
Used iPad 2 - runs latest iOS, but no Retina display (http://www.amazon.com/gp/offer-listing/B0013FRNKG/sr=/qid=/?ie=UTF8&camp=1789&colid=&coliid=&condition=used&creative=390957&linkCode=ur2&me=&qid=&seller=&sr=&tag=setlist-20)
Used iPad 3 - Retina display and last iPad to use 30-pin dock connector (http://www.amazon.com/gp/offer-listing/B00746LVOM/sr=/qid=/?ie=UTF8&camp=1789&colid=&coliid=&condition=used&creative=390957&linkCode=ur2&me=&qid=&seller=&sr=&tag=setlist-20)
I don't know if these are available outside the US ... maybe someone else can comment on how this works in other countries.
P.S. If you use the links above, I'll receive a small commission.