Probably the best way to deal with sellers that misrepresent item location is to avoid them. The "US only" item location filter selects items that are listed as being in the United States; however, as you have seen, not every seller that lists items in the US ships from the US as they should.
Before ordering you should check the estimated delivery dates -- that is a good indication of where items actually ship from. Also, going to a seller's profile page or feedback page will tell you where the seller's account was registered. Although some overseas sellers do use US warehouses to ship their items (and some registered US-sellers may employ overseas drop-shippers), if a seller is registered overseas, that raises the likelihood of an item shipping from outside the US, particularly for multiple quantity commodity items. Checking seller feedback will often reveal if there are problems with long shipping times, canceled orders or other supply chain warning signs.
