I make use of web search engines to look for web history on the buyer's email address, email name, check their delivery address against county assessor real estate tax payer listing, other web forums they may also frequent... for starters. Payment can be made using USPS money order...or bank certified check.
Other online accessible databases which can help are FAA pilots and FCC ham radio....state voters. Spokeo.com database on people occasionally has 1 month subscription for something like $5.
I may even try posting my SELL as 'local sale only' to lure face-to-face. Nowadays, buyers may have a relative or proxy near me stand in as the buyer and with cash in hand. Most of my local transactions are done at the local police dept lobby or at the police firing range parking lot.
Picture your typical Las Vegas one-armed bandit. I look for multiple supporting Web information on the person, the more the better, which helps boost their 'info credit score'
If the forum you're using to sell has a 'member reputation' feature that is a huge help (not 100% but helps members build credibility).
At the risk of being accused of etho-something, I do observe the potential buyer's grammar, spelling and language abilities
At the start I am not quick to reveal information about myself (kind of a catch 22, isn't it) unless I'm reasonably confident that the buyer is somewhat trustworthy (based on much of the above checklists). I may (then) share information which they can verify from public databases (mentioned above), my ebay member name, any web forums I moderate or a member of,. etc. I always send items insured and with signature release.
I rarely use online paying services for big ticket items. No Zelle, Vemo, bank routing number, Paypal etc. Just too many escape routes for scammers or crybaby buyers who can manipulate for a refund based on fake claims.
...anyhoo just some safety features that come to mind that I use myself.
