Also, why can't ebay set up a cart system for BINs like every other online shop? Once you've finished your shopping you pay for what's in your cart. If you don't pay, there is no sale and the stock stays in the store.

There is a system for BINs with "instant payment required", you have to pay immediately; until you pay the item is still listed and may be snatched by another person.

But eBay is not like any other on line shop; eBay is just an advertising medium and you are not buying from eBay, you are buying from different sellers.