If the seller has a good feedback history and high number of sales then I would say its a safe sale. Look at comments left by previous buyers too. I bought a Suomy helmet on eBay for $200 less than I could find anywhere else online and it was legit.
'13 Ducati Monster 796 ABS (CURRENT)
'08 Ducati 1098 (SOLD)
'05 Ducati 800SS (SOLD)