ALL bidding, BIN, offers etc are done on the eBay platform, not off of it. If someone is directing, you off the site to buy or have a link in the listing directing you off the page, it is against eBay rules. The listing should be reported for directing buyers off the site. I have seen listings like this, sure in other places but mostly model railroad stuff.
If you found something not being listed on eBay and are being told that eBay will handle the transaction, it is a scam.
If you are being asked to pay with gift cards, it is a scam.
