If you can get him/her to flare, or even get a picture where it is not lying on the bottom it would be easier to determine, however the unknown age may make this juvi hard to determine in the first place. A rounder belly, egg spot, beard/no beard, and length of fins usually contributes to the gender :)
Keep the water very clean, warm (78), and feed high protein foods for the baby. Any wholesaler who tosses babies into stores hasn't taken care of them, and being thrown into such situations can cause a shock in the immune system for some young bettas.
Breed for the breed, not for the money; the words any REAL breeder would understand.
You must know nothing in order to know all, for all is nothing and nothing is for all.