I usually buy novelty items off Ebay, but I did buy an orchid there in 2013. But I researched first, finding out the background of that seller. Like David, I try to avoid overseas sellers, so the one I got is a US seller, and actually has its own web presence too. I guess they are just using Ebay as another avenue to further promote their site too. So far that plant is still alive to this day, it is happily enduring me.
Sometimes I use Ebay the other way around, if I find the plant for sale there, and the seller info is there providing further links to their actual separate online site, then I go further to research about them and what other plants they have. It often happens, I start with Ebay and I end up with a different plant from same seller.
Reading the comments left by other customers, the rating given to the seller is also worth reading.