most big box stores do not sell bettas that young. customers don't want juvenile fish that have less colour and less finnage. your betta looks larger than 1/2 inch, more towards 1 inch or maybe a bit bigger.
i would hedge a bet that your betta is female, and is very very stressed. those horizontal lines are called 'stress bars', and show more easily on females than males. her colour will come in as she gets bigger/older. baby fish are not coloured up like adults because they are easier food targets for larger fish. the 'blander' the fish looks, the easier it can hide from predators while it grows into an adult fish that can more easily defend itself or get away.
the fool proof indicator on male or female is on the underside of the betta, between the ventral fins. if you see a small white nub, then you have a female. this is the ovipositor, part of the female reproductive anatomy.