Generally one of two cases.
But I'll briefly mention a third: Go-backs. When you return an item, well for any reason, they don't bother noting the reason, and if the reason is "defective" they do not take a swing at fixing it. They throw it in a bin with all other go-backs, and auction off the bin. People buy these bins and resell the items on eBay. Since they are one-offs and not a continuous supply, eBay is the better platform.
Counterfeit goods vs the real McCoy
If you buy an item from Amazon Proper, meaning "Sold by and shipped from Amazon.com", you are fairly likely to get a genuine, legal product licensed for sale in this country. Generally, Amazon Proper has the same quality filters as US retail stores - for instance City Electric will only sell you LED fixtures that are UL listed and pass FCC Part 18 requirements, and usually so does Amazon Proper.
Whereas on eBay/Alibaba/Amazon Marketplace, you get several things, because many of those genuine, legal goods are made by contractors in Asia using subcontractors.
- Someone sets up a factory to make counterfeit goods, possibly using stolen plans or replica dies (the subcontractor who made the dies for the manufacturer's contractor, makes a few extras for his buddy who sets up a counterfeit line).
- QA rejects from the genuine article's assembly line that have been dumpster-dived.
- Genuine items that have been stolen from the factory.
Since the QA and stolen items are "one-offs", eBay is the better platform.
Amazon Prime shipping vs. ePacket delivery
For people with enough money, Amazon will cheerfully give open access to their warehouse infrastructure, even for companies who don't want to be part of Amazon Marketplace (the "wild west"). Someone like FMC Mining might use this to ship specialty drill bits to their 9 customers. But if you do want to be part of Amazon Marketplace, your item qualifies for Prime Shipping -- even though it's Marketplace! And that is attractive to many consumers. However, this "platform access" is very expensive. It is particularly harsh on small/inexpensive items.
If you're like me, you tend to look at mainly/only items with the Prime delivery feature. The sellers of cheap Cheese junk know that. So they pay the premium to have their products in the Warehouse/Prime system, and tack it on to the cost of the item. So it's quite a premium. Yes, this is you paying for Prime. Often the same seller (looking at you, uxcell) will also offer the same item on eBay.
In that case, it ships via "ePacket delivery" from China, which is super-cheap. This is a weird service where the US Postal Service actually has a presence in Shenzhen (right next to Hong Kong) and these "ePackets" are essentially delivered right into USPS hands. Perhaps, like TSA Preclearance, they also clear customs - a lot easier to clear "the usual daily truckload" of 2000 packets of entirely LED lights at the truck in one batch, rather than one at a time in fifty US ports of entry.
EBay items are typically ePacket.
The contrast in price between ePacket and Amazon Fulfillment Services is enough to give you whiplash, and is enough to explain the price difference between eBay and Amazon [Prime].