Some things to check:
Reviews, of course. But it need to be on the list. Pay attention in particular to the most negative reviews. Some can be filtered out due to bad buying choice or such. But if several people say the sound is crappy or that it died within 6 months, move on. (And never buy the cheapo brands. Colby and all that.) If there aren’t many reviews for a model since it’s new, check on reviews for similar models by the same maker.
The specs. Find and download the product manuals. Make sure it has the connections, features, etc. you want. Be completely informed.
Make a decision on your ethics of going to a store and look at the models and then buying online. Don’t want to start a debate, but be aware this is a decision you want to be comfortable living with.
I’ve bought one from Amazon. Smaller one, but it went okay. I know others who’ve done it. Lot’s of people do. A small fraction have a problem with delivery or some such. Life is full of small risks.
Amazon is quite good at taking care of problem returns for an item of this scale.