your condition is comparing against a value of 0:
$row_idSellers['mem_type'] == 0
only if the mem_type equals 0 will it show "Buyer"
any other value besides 0 will trigger it to show "Seller"
if the values being stored in the mem_type are either 1 or 2, then you need to use one of those values in the comparison, by using 0 for the comparison, you are telling it to always write Seller.