A scammer puts up an in-game Animal Jam item (or several) for an amount of money (example: $25 USD for a for a Black Long Collar). Someone pays the seller $25 for the Black Long, but they never receive what they paid for. The scammer may delete their eBay account, making them nearly impossible to track.
A scammer may require your AJ password in order to transfer the item(s) to your account. Instead of giving you what your purchased, they steal your items.
How to avoid this scam
DO NOT BUY ANIMAL JAM RARES USING REAL MONEY. (Also, don't give anyone your password.) Not only is there a risk of getting your money scammed, it's also against Animal Jam's Terms of Service.
Sellers get banned on AJ, and you will most likely get banned if you're found buying rares. Buying rares is a risky, illegal, and dishonest way of getting them. You could lose your real money, and you could lose your whole Animal Jam account.