3. Actually only some categories allow commingling and surprisingly only the largest sellers tend to use it. The reason they use it isn't because of the inconvenience of labeling items inhouse, it's because if a claim comes back to them as damaged or other issue they can point and say "how do you know it's mine?" and they 'win' the dispute with CS. FTR Amazon claims it sickers all commingled inventory to track back to FBA sellers, I have my doubts as internally they use the UPC for scanning. 4. This is where .UK is nice. All seller pages have business listing info as well as VAT numbers.

Last point, a lot of the product you buy on A is sourced at those very same stores.