Yes, it's illegal.
Saving a couple bucks to break a multitude of laws is hardly economical. Encouraging such behavior is not acceptable.
People can and do face criminal charges all the time for improperly importing and handling plants.
You should visit a port of entry where mail is handled sometime. There are huge operations where fruits, vegetables, plants are dissected and catalogued. Any time plant matter is detected without proper documentation, red flags get sent up in multiple government systems. Repeat offenders receive special attention... to say the least.
If you think it's easy not to get caught? You're mistaken.
So you are saying it is illegal to buy plants on Ebay that are imported?
If I just need a small amount of plants it is more economical for me to buy on Ebay.