About a year ago I bought an
80cc HT kit from
"sun-thing28".
At the time the price was good, plus I was getting snubbed by US sellers who wouldn't ship to California, so I took a chance with a Chinese vendor. Back then he was quoting 60 days shipping, and I think it showed up on day 59, complete and intact, with no hassles with USPS or Customs. Quality was typical China Girl, and it's still running today with no breakdowns.
As long as you use PayPal you're pretty safe. The main problem you'll run into is shipping time. Last time I checked you have 45 days to file a refund claim, and on day 46 you're out of luck. In fact, about 6 weeks in sun-thing28 sent me an email saying "please don't file a PayPal complaint. I promise you'll get your engine." And I did. But if it never showed up I would have been screwed because it was too late to file a claim. Now I see sun-thing28 is quoting "20-25 working days" which is still 4 to 5 weeks, so it's better, but still cutting it close.
eBay and PayPal have built-in protection so there's really no risk as long as you know the limits. If the vendor has good feedback, personally the only thing I'd worry about is shipping time.