Its a gamble.
Ive seen some shocking cheap import spacers, but ive also seen some equally cheap spacers made by a local engineering company, built with the high quality materials and perfect tolerances.
But how do you know where they have come from if buying from ebay? (unless you really trust the seller)
Only real way is to know they are built properly is to buy branded (expensive) spacers.
Like i say a gamble, but think what your gambling with.