Decide if you need/want a 7k or 10k trailer. A 7k trailer that weighs 2k lbs can haul roughly 5k lbs max. But who wants to max out their trailer? Not me.
Get one that has ramps that store underneath.
Decide if you want a a 16', 18', or 20'.
Electric brakes. Some nicer (and usually 10k) trailers have brakes on both axles.
Do you want aluminum or steel?
Figure out how you plan to secure your typical load as well as other things you may haul. That will help you decide what kind of securing points your trailer will need.
Check out some different brands. I found I like H&H because the components are high quality and it makes a different. I ended up getting a 16' 10k steel H&H trailer which ticks most of the boxes for me.
If you buy used make sure the deck is straight. I see a lot, and I mean A LOT of bent trailers.