Ball pythons actually prefer smaller cages than other species of snakes, because they hide in rodent burrows and termite mounds, in the wild. So, a 20 gallon long-sized tank is fine for a snake his size.
He should be fed a small rat every 2 weeks, (Or 2 or 3 mice a week), at his age and size.

Additionally, I’m sorry to say, but you’re unlikely to get more than $50 for an adult normal male ball python. If it’s proven to be female, you could get perhaps $90 for a girl that size (but people want bigger females that weigh at least 1500 grams, so they can be bred). Folks often overprice their normal male snakes in ads, because they don’t realize that they can’t get what they put into them in food and housing. The market simply doesn’t support it. Retail prices are not what you can expect to get when you re-sell your pet snake.
Females and morphs are a different ball game.

I recommend you look into what breeders are charging for male snakes, and adjust your pricing accordingly (at least, if you really do want to sell him).