As far as I can tell, realistically only three account types people want and for two reasons.
PK builds - no explanation needed. There are many builds people want to try. Lengthy training for pures. Usually sites make these accounts, so not random people. Low recovery risk, plus people keep wealth on main.
Mains - usually bought by botters. If it can do Zulrah, Vorkath, Blast Furnace, that's great. These accounts are cheap, look less bot-like which helps with less reports at the very least, probably heuristics to some extent as well. You'll make more than you lose, even once you get banned, due to cheap rates. Unless you buy from someone on Sythe, pay BTC or GP and he recovers 2 days later, you'll have had no time.
Stakers - self-explanatory.
Fourth reason (uncommon), some guy wants to skip the grind, but rare .Too high risks as websites don't create high level mains (going beyond combat), only resellers. No point buying an account when there's cases of recoveries years later. Also the entire game is a grind (so you skip the game essentially) and endgame content isn't that great, maybe if you have an ambition for Inferno or don't realise the recovery possibility.