I've bought a Ferrum Forge Tredecim on ebay for 30% off the retail price, and it's a rare bird at that. I had no issues with it.
If you're buying on ebay, exercise the same precautions as anywhere else. Look at who's selling it, their feedback score and join date, inspect the pictures, send them an email/message for ore info, etc. The risks of buying things online are real and present no matter what site you're on. To be buying online, especially from third parties (not makers or retailers), one needs to accept that there's a chance that things go awry, and then one needs to be vigilant and try to stick to sources that seem trustworthy.