I want to sell new electronics on amazon. What happens if the stuff I've sold is defective? How do I keep from losing $?

Generally middle-men have to add some "value-added", like, checking the items to make sure it works, taking support phone calls, accepting returns, etc. That's the main reason customers come to you and your higher prices rather than going directly to your sources. You have to build in your costs for support and returns into the prices you charge. For example if you suspect 5% returns, you have to raise your original prices by 5% to make up for that.

If you're getting your merchandise from a place with "14 day" return policy, then either they're getting "gray market", "returned" or "refurbished" products without the manufacturer's warranty. You pay less for those, but you and your customers lose the protection of the warranty. It's probably a high-risk concept to try to resell those kinds of products.

As you've figured out, someone else is making a profit on these items, not leaving a whole lot for another level of merchandising.