I would say it's fairly easy. Was doing it last year and early this year with moderate success. Walmart was a better option for me than Amazon.
The hard part is, you need to keep checking that products don't go out of stock or change price. The price changes can even happen 2-3 times a day! You can imagine how hard it can be, when you have 50, 100+ products listed. But there are some paid options to monitor your links.
Lots of times, the package that arrived to the customer was missing only one part, example:
"I had bought a vanity from you dec 13 2014 a vanity my daughter was moving and we just put it together and find out a piece was missing the back that hold the mirror to the desk is there a way to get that piece."
These kind of things are a pain in the ass to manage, not to mention all the returns.
What made my account to go below average and this means your Ebay career is basically over is, I was selling some lights and the vendor I got them from screwed everything up - the lamps just didn't arrive to the customers. I got lots of complains and "item not received" and negative feedbacks. My account went below average (and when that happens, you're screwed). Of course, you can buy stealth accounts, but it wasn't worth it for me.
If you want to try it, make sure!! you never use products from other merchants on Amazon, only use products that Amazon sells or fulfills. Also, if using Walmart, make sure you never sell anything that is a "Walmart exclusive", because when they sell the product out, you won't be able to find it elsewhere.
Another option to consider is Wayfair, you get cash back from all purchases.
