Someone I know made easier money by listing Amazon-available products on eBay and selling them there for higher prices. They didn't even need inventory, they just got them with Prime shipping after a purchase on eBay. People are stupid.
On eBay before buying stuff I always ask the seller if they can beat the Amazon price - 5% cashback that I get on Amazon, or deduct CA sales tax in their price if the product is available on B&H. If they can't beat that I buy on Amazon / B&H.