Security
Phishing is the real threat. The defence is three quick checks.
The subject of security on a Tor market reduces, for a buyer, to one threat above all others. Not the market vanishing. A lookalike address that copies the market and harvests the password you type into it. Phishing is how most people are actually robbed here, and the defence is a habit rather than a tool.
How the attack works
An attacker stands up an onion that differs from a real one by a couple of characters in the middle of the string, where the eye does not catch a substitution. They copy the login page exactly. You type your password and it is theirs. Some clones go further and ask for a recovery phrase at login, which no real market does.
The three checks
First, copy addresses from a signed list and never type one from memory. Second, when the login captcha prints the market's own address into the image, hold it against your address bar and confirm they match. Third, cross-check the address the header prints. A clone can copy a theme easily. Faking the correct address in two independent places while sending you somewhere wrong is much harder.
The habit
None of these takes more than a few seconds, and any one catches the ordinary attempt. Run them every session, not just the first, because a months-old bookmark can point at an address that has since rotated out and been picked up by someone hostile. Cross-reference the desk article on verifying a mirror.