I haven't bought very many parts from China, but most have been OK.
I have noticed that the IRF640 Mosfets, Don't fully meet Spec.
But OK for Non-Critical applications especially to their voltage rating.
Upon sending a Complaint, I received a Full Refund, and no need to return the parts.
Most of these Vendors want Happy Customers.
Digikey wants 98 cents each for quantities up to 100 pcs, Plus Shipping Cost.
From China, one listing is 50 Pcs for $12.00 and Free Shipping.
Some others are around $22 for 50 Pcs.
Although delivery from China is usually a Month or more.