Buy a Second-Hand Item Without Getting Burned

Private seller listings have no returns policy. The five-minute check at collection is the only quality control you get. This tells you what to look for, the red flags that should make you walk away, the fair price range, how to negotiate without souring the sale, and the known issues specific to this type of item that are worth testing on the spot.

I want to buy a second-hand {ITEM} from a private seller. Help me buy it wisely. Tell me: the five most important things to check or ask before agreeing to buy, the red flags in a listing or seller's communication that suggest something is wrong, the fair price range for this item in good condition, and what to say to the seller to negotiate without being insulting. Note any common known issues specific to this type of item that I should test or inspect.