From what I have been seeing. it looks like Amazon has been started to ban people in the last 6 months. If you are still returning a lot of items, I think there is a fair chance of getting a warning and then a ban.
That seems to be the case. I would bet it's partly due to investors leaning on them to become more profitable. Amazon generates obscene amounts of money each quarter but are either losing money that quarter or just above break even. Jeff Bezos said many times that Amazon can be extremely profitable anytime it wants.
Shipping kills Amazon even with standard free shipping so any returns by customers impacts their bottom line. There's a rumor that Amazon Prime will go from $79 to possibly $120 in the future and even as a Prime member you'll now have a minimum purchase in order to get 2 free shipping.
Newegg at one time was a lot like Amazon and got beat up and lost a lot of money so now they charge restocking fees. They'll even sell you New items that at best are open box or used.