I was disturbed to read about the bad experience that a fellow ebayer had with a 'generic' brand ATV, purchased for his son. If you're concerned about purchasing a 'generic' bike, then I'd like to point you in the direction of 'Atomik motorsports' (do a search in ebay stores and you'll find them).
I'm a tiny grown woman (5ft2" and 45kgs) and an adult bike was far too big for me! My husband and kids ride so I thought it was time for me to start too, but I didn't want to spend big $$$ for my first bike - so I bought a 125CC pit bike from Atomik - and we've had absolutely no problems mechanically (some damage through my own bravado, but that's another story!) My best advice, as with any ebay purchase, is to CHECK THE FEEDBACK of the seller. I've been rapt with my 'little' bike, and I would certainly recommend an Atomik to those considering buying a generic bike.
I'd like to add that we are NOT representatives of Atomik and other than our ebay dealings we don't know them at all - we just wanted to share a good experience.