To find stores with the most amount of parts from a wanted list, go to Want > Buy, choose your wanted list(s) from the left, and select Find Stores. From here you can filter by country, payment type, and the feedback level of the seller.
To find stores with only a subset of parts from a wanted list, choose the wanted list and check the left hand side checkbox and then click Buy X Selected (where X is the amount of items you have selected).
Clicking Select next to a store will bring up a confirmation modal where you can choose which items to add to your selection. This modal also shows the minimum buy of the store as well as the location. When you’re happy, select Confirm Selection. Continue doing this until you have found stores to buy all your parts from and choose Add Selected to Cart. You cart (in the top navigation bar) will now list all stores in which you have items in your cart; you can click on each one to check out whenever you’re ready.