you can usually accurately loupe sex a gecko which is 3 inches SVL (snout to vent length), and sometimes younger if you have very good magnification and experience looking for pores, but a bulge may not appear until a male is much older. the gecko you are looking at is much too small to sex, as it is likely only a couple months old. petco's price is good for any gecko, male or female. if you like the looks and it appears healthy, go for it.
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4.1 snakes, 4.6.1 crested geckos, 0.1 gargoyle gecko, 0.0.2 red-eyed tree frogs, 2.0 devon rex cats, 1.0 betta, 0.1 sun conure, 1.0 lovebird