I have purchased cards from China via eBay multiple times and never had a problem.
Having said that I do see some unusual behaviour sometimes. For example, there is one seller selling several copies of the same serial numbered cards (/49) or less. The cards also always seem to go to auction, "sell", then get relisted a few days later.
Is it possible the one seller has say 7 of a /49 card or 5 of a /25 and are trying to push the market up? Sure. Is it also possible they have counterfeited these cards? Sure. Though they aren't worth that much.
Of course you can find similar dodgy actions from sellers anywhere in the world.
It all comes down to common sense. Check out the seller's feedback and auction history then make a judgement from there.