Hi Ben, I purchased my Samyang f2.8 UMC off eBay four years ago and had no problems with the purchase. The seller was a Chinese vendor based in Hong Kong. Obviously this is only a sample of one so cannot say that others may, or may not, have issues but at least my experience was a good one.
As far as the lens goes, I have been very happy with the purchase, it performs well and has met my expectations for wide field imaging. I use it on my Canon 60Da DSLR.
Here is a link to an image taken with this lens.
Hope this helps.
CS,
Rodney