The people selling them fall into two categories, those that genuinely do not know what's wrong with the item, and those that do.
Be very careful, take a while to look at the sellers other items, if they normally sell that genre of item in a working condition, then it's a fair assumption that if they naively describe their faulty unit suggesting they have no idea what could be wrong with it, then they could be telling porkies. Chances are that they've had the back off it and found it to be completely buggered, or an economic write-off, i.e. the repair will cost more than a working unit is worth.
Remember, modern electronic consumer goods are generally not repairable by your average screwdriver jockey, they contain tiny surface-mounted components which often have internal logic in the form of software and firmware.
Unless you know what you're doing you may well end up with someone else's junk.The 'Naive' approach to selling junk....... Use basic text, make deliberate spelling mistakes and mix in a bit of text speak to make it look like they're a bit dim. Use soundbites like, 'worked last time I used it', 'don't know nothing about these', and the daddy of all phrases, ' UNTESTED'. Why wouldn't they test it? They probably did test it and it was fecked.
Also, they may play on common knowledge of known faults, and include that common fault in their listing for the item which they know is terminal. Such as the 'three rings of death' on Xbox-360's, they will mention the TRoD, but forget to mention that the rest of the innards are toast, so a potential buyer thinks they only need to buy an aftermarket cooling system, or re-solder a few joints and they have a bargain, wrong, they've bought a pile of junk, and there's no comebacks.
Don't forget that although most sellers of faulty goods put something to the effect of 'no returns' on their listings, that's bull, they still need to provide an accurate listing, if they advertise a laptop as 'black with broken screen', and when it turns up it's pink, then normal eBay rules apply and you can still make a Paypal claim for item significantly not as described, despite their disclaimers.
Check the seller and their buying and selling habits.
Make sure the format of their listing for faulty goods is consitent with their normal listings.
Do the usual Google checks for the item, i.e. cost and repairability (not a real word).
Treat the item as completely knackered despite the sellers claims, and bid accordingly.
Don't be put off by their non-returnable disclaimers, if it's SNAD then make a claim.
Unless you're an expert on the faulty item for sale then bid as though it is for spares.
For every 'faulty' bargain there are twenty lemons.