If you're in a decent sized city, you should be able to find a local buyer on Craigslist or FB Marketplace... Beyond that, for higher value, smaller items like your M3 Ultra, I would talk to your local police department and/or library to see if you can do the exchange there. Larger libraries usually have a police officer on site or nearby, and the PD office near you may also provide a "safe" exchange location... I'd bring a monitor/keyboard/mouse so you can demonstrate the system working properly.
YMMV but between your nearest PD office and Library, you should be able to use one or the other for your exchange of goods/money. The biggest thing I've sold is a mid-range video card during late covid (I managed to get a better one via newegg shuffle) so I sold the old one (RX 5700XT -> RTX 2080) to make up the difference a bit. I just did the exchange at the Starbucks near me for that.