A buyer should be able to know where the seller is located, and to bypass listings that are being shipped from another country. You have this option on ebay. I have noticed that Tradesy has recently added expected receipt dates so you might be able to guess which sellers aren't US based.
The ETA may not serve as a good indicator. Some sellers have to wait for the shipping kit before shipping out the item to the buyer. Last time I bought from a seller who's 2 states away and the ETA given was 14 days. And it arrived now the 12th day in one of those fancy Tradesy boxes.