I think there has been a huge increase in sellers gaming every system they can. ebay, amazon, etsy, poshmark, shippers, etc.
I've had several recent ebay purchases come via Amazon. Some get here really fast, some well, still waiting on some.
Looks like many Amazon Prime members are using the prime "free shipping" on all orders to offer lower prices on ebay, then they order the item for you from amazon. I open the package and find a gift receipt from the Amazon seller.
country of origin at ebay isn't always correct. That's up to the seller to specify and some just lie about it. A recent item listed as shipping from "United States" took 3 weeks. Tracking showed it came from a shipping company, probably in China, to New Jersey, then put in domestic USPS to me. 3 weeks is fast for a container ship from China to New Jersey, maybe too fast. Air freight in a huge box,then resorted and mailed? It had 2 labels, 1 to NJ, then an overlay to me. One sign it's coming from Amazon is ebay messages initiated by the seller that it's "on the way". Normally ebay sends auto messages when the tracking number is assigned at the time of postage purchase, then "out for delivery", then "delivered".
Last year I got an ebay notice an item was delivered. not. I checked the ebay supplied tracking and it showed delivered. I went to the local USPS because I know they have more detailed tracking and it showed a completely different address in town. I messaged the seller and never got a reponse. Two days later the item arrived with a completely different tracking number on the package that had no tracking history on line. It had an Amazon gift receipt in the box with an Amazon listing number. Sure enough same item for 25% less at amazon. I tried an amazon search for the same description but the new product didn't show up under the sellers name. Leads me to think there are many amazon items that aren't public. Sellers are using Amazon's services but don't allow public sales.
I've recently started adding a line in my ebay for sale items that I'm a real seller and ship from my home town. No fake store fronts. All my items are what you see. No stock photos from the web.
PS, number of items sold by a seller are a hint too. Just a few items with less than 100% rating is usually someone selling without a clue. First time sellers are usually a disaster.