In that case, you probably have to:

• Use Tor respectively Tor Browser (without any additional extensions, and set the default security level to “Safer” if possible with Facebook) to create a completely new, anonymous account on Facebook. Don’t enter any data there that could be linked to your person, e.g. no real mail address (use an alias, ideally a completely new one), no real names, no real data, also no real billing or delivery address. Literally nothing that could be linked to your person. Only ever use Facebook’s site within the Tor Browser, to ensure that your real IP address and browser data aren’t leaked. Never use their apps, never use your regular browser for it, also don’t use PWAs because that’s similar to using a regular browser, which reveals your real current IP address to the site. Unless you use a VPN to have a different IP, but you’d have to minimize your VPN usage just for that app interaction. If you continue to use the same VPN IP for other stuff as well, you could de-anonymize yourself later on. Mullvad or Proton are commonly regarded as good choices for trustworthy VPNs which don’t log or sell any user data, or at least there are no known cases for it (yet).
• Don’t add any friends on the site, try to limit your interactions with the site so that it can’t create a big psychological profile from you and try to link that behavioral data to existing persons (the more you use the site the easier this method might become for them). Behave slightly differently than you would normally.
• When you buy something, remember that you’d have to conceal your real delivery and billing data/addresses as well. Which is hard to do when you actually want to buy and receive something. Your payment data and/or address data can EASILY and instantly de-anonymize you, also in front of Facebook. So my suggestion for something still practical would be to have a relative or friend buy it after you arrange that with them, have it delivered to their address, and you pay them for it and gather it from their place. So in essence you need a proxy person to do the receiving and paying for you. If you want to sell something, that’s more inconvenient of course, but you’d also have to do it similarly.

The most problematic de-anonymizing data about you is going to be your real current IP address (which is revealed when you use a regular browser, PWA or their app, all with a non-VPN or non-Tor IP address) as well as billing or address data. In case you’re using their app, they’ll receive even more data to de-anonymize you more easily.