bring a letter scale into the store with you. The sprues will have different weights to them; the scale will be able to pick up on them. It worked for yugioh cards, able to pick up the presence of a card sized piece of foil; it will be able to clearly tell the marines apart. If the store owner complains, just say that you plan to buy several and do so. We could almost get a chart going of the different weights once they are on shelves.

As long as you buy a different weight each time, you can get the full team with no repeat.