[mseebach]

If you're buying something expensive, require an original invoice and check that the serial number matches. If someone is apprehensive about that, somehow they lost the invoice for the brand new unopened PS4 they bought last week, or similar, just drop it.

If they're selling close to the original price, just get it from a shop instead, that way you'll get warranty etc too (there's generally no warranty on goods with "lost" invoices).

In general, look out for suspicious behaviour. Are they willing to meet with you at their home or office? Do they readily share their phone number? Do they have many excuses (one of the most common tells for liers are an excess of over-thought out excuses) for not doing things transparently? If it's too good to be true, it probably isn't true.

[throwaway613834]

Thanks! The trouble is all of these can fail for what I would consider legitimate reasons. Requiring a receipt fails if they just open it, maybe even use it a bit (or not), and wait a few months before selling it, since nowadays people seem to genuinely go through some stuff (e.g. phones) pretty quickly and not necessarily keep or have receipts, especially if they themselves bought it second-hand. Meeting at my home or office is not something I would ever feel comfortable doing, and is generally recommended against when purchasing online. Sharing phone numbers is something I'd only do when the decision to purchase has gone through and we're actively trying to meet up (obvious privacy concerns), etc.

[mseebach]

It's not an exhaustive, binary checklist, they are signals to watch out for. They are basically surrogates for trust. If you're selling something that looks like it might have been stolen, you have to come up with a compelling reason to trust you. Meeting at your office and sharing a business card might be such a reason. If the only thing you're comfortable with is meeting in a deserted parking lot late at night, and only communicating over a throwaway anonymous email account, the counterpart is right to suspect something is fishy. On the other hand, if you have an original invoice from the Apple store, then it might be OK.

Sure, there are legitimate reasons for not getting any of these right, but if they get all of them wrong, be very careful.