First make sure you know what he is (pygmy or runt of the litter of ordinary beardies).
Second make sure the person selling has a bonefide keepers' permit (if not walk away as it's likely wild caught).
Third make sure it's eating well (insects of appropropriate size and veg and greens and softened pellets maybe) and has been given Ca and Vits.
Lastly check out how it moves in an enclosure and maybe on the floor.
Maybe a vet check.
Hey , even if it's undersized for it's age and maybe the runt of the litter it can still be fabulous family pet and with a little love and TLC and a good diet it can still grow to be nearly as big as it's luckier siblings are and you'll know you are giving it a good life.
You're better off buying from a breeder , but sometimes people find they loose interest in the pet or can't commit the time they need when hatchies , juveniles (multiple feeds per day) and they'll sell the pet and it's kit so you get a good buy.
If it's not been getting greens, it can be handfed them to get it to associate the greens with being food and a little perseverance will eat them OK.
Best advice I was given before I got my first lizard was DO YOUR HOMEWORK and PREPARE EVERYTHING you need before you get one.
If it's the one I saw on GumTree , there is a lot wrong with that enclosure , lighting, bedding being the most obvious (these are easy fixed).