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.
