well first of all i suggest you learn as much about ball python care as you can before gettin him but thats a given, there are a few obvious signs to look for health wise, is he/she retaining any shed? do you notice any scale rot? does the snake have any kinks in his/her back, does he/she look dehydrated, etc ... etc.. if you already have a ball python then you should know what to look for(not sure if u said u had one already) another thing is a vet can get kinda pricey even just for a check up so unless you notice anything seriously wrong with the snake then i dont think it is necessary IMO and if there is anything wrong with the snake maybe you shouldnt be purchasing it in the first place, if u are worried about the snake and dont mind payin for the vet bill then there is no harm in gettin him/her checked out not to mention if it is ur first snake u can always take it to the vet and talk to the vet about proper care and what to look for next time you purchase a new snake. . . hopefully this helps and there are alot of websites and books out there are ball python care and what to look for when purchasing a new ball python just check them out and good luck!!