Ultimately, whether or not a used dirt bike for sale on eBay can be trusted is down to the buyer's discretion. Some ways a buyer can judge a seller's trustworthiness are to read his seller reviews, ask the seller questions about and arrange to view the bike in person.
The seller reviews on eBay are the primary way in which a buyer can tell whether or not a seller should be trusted, as all reviews are written by site users who have successfully completed a transaction with that seller. This ensures that there are no false reviews written by third parties. These reviews cover all aspects of the sale, including the buying process, the products sold and any experiences after the purchase. If a user has poor ratings or many reviews that mentions aspects such as inaccurate product descriptions then it is likely that his listings for used dirt bikes can't be trusted.
Another method for telling if a dirt bike can be trusted is to actually test the bike before making the purchase. A buyer can contact a seller and arrange a meeting to test out the bike, then make the purchase from a mobile device before acquiring the bike.