Kind of a vague question, but...
If you saw a vehicle on another site, and after contacting the seller, they stated that Ebay will be handling the transaction...
it's a scam.
The correct way to purchase a vehicle is to, after winning the bid or buying it on eBay, go inspect the vehicle and the title, and if everything appears to be correct and everthing appears as described, you pay the seller.
You do not pay for the vehicle sight unseen, most descriptions are biased, so to avoid any issues you go and inspect the vehicle, or hire a third party to inspect it for you before you pay.
Things to watch out for:
No legitimate seller sells a vehicle to a complete stranger for thousands less than the wholesale price a local dealer would pay.
No legitimate seller offers free shipping, an inspection period, and free return shipping.
No legitimate seller attempts to sell a vehicle on another site and says that Ebay will be handling the transaction.
Ebay does not have agents.
Ebay does not have a warehouse.
Ebay is not a shipping company, they ship nothing.
Ebay has nothing to do with other sites.
Ebay does not condone the use of Western Union, Money Packs, Gift Cards, Money Gram or any other unsafe method of paying.
Ebay is not an escrow company, they do not hold anyone's money.
If you see a vehicle on the Internet that you are interested in purchasing, make arrangements with the seller to inspect the vehicle and the title, or hire a third party to inspect it for you. If the seller states for any reason that the vehicle can't be inspected, then walk away, the vehicle does not exist.