wow.. that is completely gross. Well lets say hypothetically, this seller did do it. and even though it is probably too late to figure out, I would have done exactly as you did but I would've taken one of the ants and took careful notes as to what kind it is. Identifying an ant and then using the intarwebs to see if it is native to the state I was living in. If it isn't and then it's clearly one from the sellers state or from in-transit.
I guess this is something you can look back on and laugh about.
I guess consider yourself lucky it wasn't roaches..