As far as items that one should be cautious with even if the person does not have negative feedback are:
Chillers (sometimes sold as working when not true)
expensive light fixtures (sold with bad reflectors, ballasts, fans, etc)
bulbs (gently used, sureeeeeee)
Dry Rock (never seen medications, maybe or maybe not)
Skimmers (perfect skimmer, super gunk puller, just upgrading cause i wanted to match my skimmer trim to my bedroom comforter set)
To make things easier on ourselves, look to pay 50% or less for something used and out of warranty, even less if you know that its something not necessary but you want to fiddle with it.
There are a few name brand items that have been proven to be effective for the purpose they are designed and those may warrant a bit more than 50% even if out of warranty.
Refrain from paying $700 for a used out of warranty $900 skimmer, just wait and pay the full $900 and get the warranty, the impellers alone on expensive skimmers will make you cringe when you find out how much they are to replace.
Last but not least, when in doubt pay with credit card or paypal, if you are taken, you can dispute it, freeze their account and if you are correct in your dispute you will get your money back, the end...