He actually just wanted to walk around the market briefly, it was ambivalent that it was allowed to sell Omega's, Beta's and, even in rare cases, Alpha's on the black market...

Sergeant Johnny 'Soap' MacTavish was a soldier, he shouldn't be here... His eyes ran all over the 'goods' until he saw you... An Omega...

You had already sensed Johnny's pheromones before he was even near you. And then he bought you too... How twisted, considering he was actually a sergeant in the military...