I would not trust FedEx,UPS to ship floorstanding speakers no matter how nice the box is.
Light monitors have a better chance in a soft foam skeleton double boxed, dense foam can lead to issues. Heavy dense small boxes are prone to damage, heavy monitors or subwoofers are bad risky. If you can ask the seller to send pictures of packing process or boxes to be used might give a little reassurance.
Ive been buying on Audiogon for over 15 years and have never been ripped off. Knock on wood. Way back when no one was using PayPal it was all USPS money order. PayPal was used on eBay.
I find most people with good feedback are honest good people that will go out of there way to have a smooth transaction.