@grislybutter laid out a number of the well-known precautionary steps, all of which I agree with.
I’ve purchased 4 or 5 pairs of 2-way monitors/speakers and had very good luck. I followed those precautionary steps, plus one other that needs to be discussed: protected transactions. And that usually means either Paypal’s "Goods and Services" category of transactions; or using a credit card; or both.
I really dislike Paypal. It’s a parasitic organization that siphons off ~3% of each transaction, more than some credit cards. They also burn you down on currency transactions, which if/when you buy from another country (in my case, Canada); in such cases they make exchanging currency unavoidable and offer rates considerably worse (ie, more profitable to them) than any bank. .
Another factor must be mentioned regarding Paypal: there is an avalanche of ignorance out there about the IRS’ planned (not yet implemented) program to tax some (not all) of Paypal’s "G&S" transactions. The net result of this is a LOT of sellers proudly demanding Paypal payments only via "Friends & Family," which is an unprotected transaction with huge risk.
But I digress. When I buy speakers, I insist on using Paypal’s "Goods & Services" feature. If that’s not acceptable to the seller, I walk. I also save all Q&A in my transaction w/that seller as a pdf file so I can prove (if necessary) that, for example, they said the drivers were intact and functional, yet they arrived non-intact and non-functional.
Finally, I’m not a fan of face to face transactions--going to someone’s home, or them coming to mine. It feels risky to me. But the only time I made an exception for that was one time buying a pair of vintage speakers (mid-’80s vintage KEF 103.2s). I needed to see and hear them in the seller’s system.