Can I buy the oakvale tavern at any point in the game or is that a building..

You have to kill the owner to buy it mate.

Once, like a year ago I was playing my evil character and I was like a one man army I raided it so much I killed like a million villagers and guards and when I left the next time I came back ALL the shops and houses were for sale.

I was awesome :D

Thats one way of doing it but if you don't want the guards after you then try using this glitch get battle charge and goto lookout point then as you're going tward the door to go in use the battle changre then hold the left analog stick and let it load after that
Cheek the guards they shuold be frozzen soldid if so then goto back to the town you wanted to buy out any what not all the building shuold be opened
For buying it's the same for all regens evryone will be forzen so you shuolden have any trubble geting what you want tho if you're goingto steal weps only the first shop in bowerstone south will be able to steal from the weo shop up pasted the tabvern will be gone so
There ya have it hope it works out for you as it did for me