Some years for a netbook? Sorry to say, but you'll probably be lucky to get a single year out of a netbook.
But if you insist... I'd rule out #1 and #4 immediately. One has stated errors, the other has no battery. #2 I'd probably leave as well, since they apparently felt compelled to state something about no problems, they're clearly trying to distract you from something else. Who knows what, but let someone else find out. If #3 is straight from an authorized repair shop, that might give it a small edge, even though I've never heard of BenQ doing anything but monitors. #5... Samsung's computer division is brand new. They may have poached a lot of former Sony employees to get it up and running quickly, but that doesn't necessarily mean anything.
Honestly, ebay is not the place to be computer shopping. You're just as likely to be scammed out of your money as you are to get a system that actually works as advertised. And if they're already used systems, you have to factor in the level of care the previous owner gave to them, which you can't do.
I want to buy cheap used UMPC for studies. I want help on choosing what to buy best (for best productivity, to not fail after some years of use). I found on ebay four variants to buy:
1. Sony VAIO VGN U50 (used, with 0271 and 0251 errors)for $210;
2. Everun S60H used, for $280,no minor problems;
3. Benq S6, refurbished for $280;
4. OQO Model 2, used for $320, without battery.
5. Samsung Q1, used, for $320.
Any comments would be appreciated.