I went through a “contractor rich” phase right after I entered the industry and went on a ridiculous buying binge. I won some, I lost some. One or 2 of the bad decisions are still collecting dust. Unless you’re strictly dealing with larger, reputable sellers you’re going to make mistakes. Just learn from them.
Do discovery. Ask around. Always look at feedback and reviews before you purchase. Take particular caution when using Gunbroker and similar sites. That’s pretty much where all my folly came from.
(EDIT)BTW- I’ve also had gun stores/FFL’s rip me off too on online gun purchases. A couple times they took components off rifles I bought and then said they were never in the box.
Always bear in mind the gun industry is rife with scumbags who know no shame.