Most things that I have cross shopped from Amazon vs eBay are quite a bit cheaper on eBay. Much more than 5% cheaper. Some things I have found to be 40% or so cheaper.
Yeah, but how much of that is the "I might not get it or it might be fake" discount?
I stopped shopping on eBay a long time ago, except in situations where it was the only way to buy a particular product, because the risk is so much higher if the product doesn't work or is damaged. Return policies are inconsistent to nonexistent, you're on the hook for the return cost (if it is even possible to return it), and at least historically, you could generally assume that they would side with power sellers over random individuals almost every time, so the company doesn't have your back.
And most sellers would take several days to ship out products in the first place, and then would ship them by the cheapest, slowest shipping possible, which means it takes a week and a half to get anything. It costs money to ship out goods within 24 hours of when an order is placed, and that's factored into the cost of buying things from major vendors instead of from some random person on eBay who happens to be hoarding toilet paper or whatever.
Buying from eBay has always felt a bit like sending your teenager to buy produce at the farmer's market, knowing that there's a 20% chance that the right farmer will even be there that day to provide what you want, and if the kid picks bad produce, you're stuck with it. And your kid doesn't have a car, and thus will have to walk there and back, so the trip will take six days.
The extra protection offered by going through a reputable vendor is worth it to me more often than not, as is timely shipping. When timeliness doesn't matter,
I'll buy it for 60% off from AliExpress and wait the extra two to four weeks.