Price is usually the first thing we think about. That is a good reference point. The next, is to determine whether it is feasible or intrinsically possible for an item to fall below a certain price. E.g. can a notebook computer be just $50? Yes if it's an old one with older specifications like Windows 95.
Can it be possible for a new mobile phone? Yes if the seller is having a discount, bought in bulk, or perhaps he misquoted the price. And the very bad answer is yes, the item was a stolen item, and thus selling at a low price to get rid of it.
So how to distinguish from the good and bad?
Simply: Go do some background research. Look at the seller's previous items sold. And the same item sold, see if it's listed for a few times. Or just once. What is the closing price range? Compare to the current market price in the shops? Shops pay rent. So factor in. Understand the history of the seller's dealings. Is it just selling shoes, shoes related, then something totally different?
From the history, the feedback points you can go easily by contacting the previous buyers by emailing them via eBay. And see their opinion.
Some sellers have 100% rating. That doesn't tell much. The seller could be just selling 200 items. A seller with a 98% rating, doensn't mean it is a dodgy seller either. Perhaps the seller had a few bad buyers instead. Remember eBay protects buyers more than sellers, in wanting to get more new people to sign up, and sellers are paying the fees NOT the buyers.
Thus: check on the history again. Perhaps the item the seller sent was actually lost/stolen and if the buyer did not pay for INSURANCE / REGISTERED Post, then the onus is on the buyer. An angry buyer out of their own fault, will still want to find a bashing potato and obviously the seller gets it.
Also the possible "oh I am sorry my daughter click it I don't want it bye" buyer. This is the worst type you find. They won't pay. Making your item stuck at least 2 weeks (eBay wants you to file a reminder and then only after 7 days for the official reminder, then after 10 days or so to get a credit back IF the BUYER agrees). If that slimy buyer says payment has been made, eBay withhold your eBay fees, your item is stuck, you end up frustrated dealing with all the filings, and your item is not sold. And you've lost say 10 hours of time (that's like $100 of worktime) to deal with this fake buyer.
WORST: This fake buyer in turns gives you a NEGATIVE rating that contributes to your fallout from 100% to 98%. Cruel? It is true. So do not judge just based on the ratings. Find out more. Find out why. eBay is bias for a reason. And it is not perfect. So even good sellers have a hard time defending their reputation.