Your best friend is Youtube. Learn everything you can about a module before purchasing it. There are loads of tutorials, demos, and people simply showing off a patch/playing with all of the modules you are considering.
Do not buy anything used from individuals. How will you know if the module is damaged unless you have experience with it? The pricing on used modules is not considerably lower than new. That's just my opinion. Your mileage may vary.
You can get a lot of the Mutable clones from dealers now. Go that route or find a maker with lots of experience/referrals.
Find a dealer that you like and check out their policies on returns. Sometimes a module isn't what we were expecting. I generally buy from one dealer with a couple of alternates for hard to find stuff. Build the relationship and get to know them on a first name basis. Depending how deep down the Eurorack rabbit-hole you go, some sage advice or recommendations can save a lot of heartache.
Most modules follow the same rules as far as what orientation the ribbon cable is plugged in. Get it backwards and POOF... you now own an expensive paperweight. Some modules have built-in protection from plugging them in backwards. Most don't. When you're setting up your case, do it slowly, confirm everything and double and triple check everything before powering it up. If you're into sacrificing small animals on an altar to bring good fortune... better break out a chicken before flipping the power switch. It's not the first time you plug something in that you fry it, it's when you're comfortable and overconfident.
That's pretty much what I've got. Take everything slowly and make the most of what you have before expanding.