Looks like a alot of fun, but the price is wrong IMO.
The 100cc one is probably regulated in your area ( most states state that 50cc bikes are unregulated...) but you could probably get away without registering/insuring...
Lifan looks like they have a good product, a rather usable dirt/street bike that looks like it would be fun. I'm partial to dirtbikes in general though, so Your Mileage May Vary.
http://www.wholesalepricebusters.com/lf150gy ( that is a distributor... I can't find lifans site right now. )