buying orchids off ebay is like buying anything really
Check the sellers profile, read the comments. You want to make sure there reliable.
Make sure the seller shows some understanding of orchids. Ask how they grows them, where they got them from.
Is the photo what your really getting? is there a ruler to prove its size?
You dont want to buy from anyone too far away, as it means the plant could be boxed up for some time. Ask how they package them up to.
Dont order plants when the weather is cold, as the plant may have little protection in the van/ depot.
Thats what i can think of so far