You can buy slayer exp in theory... If you are rich and pay a lot of money to use a cannon the whole time you will get exp faster. The skills you cant buy are woodcutting, mining, fishing, theiving, agility, kinda runecrafting...you can hire people to help which is why rcing is only kinda. Every other skill you can use wealth to help speed it up significatly I would have to say runecrafting my friend is 99 and ranked 7th and it literally took him 6 or more months of doing almost NOTHING but runecrafting to get it.