What is the best way to buy houses without killing the owners.?

Marry the wives of the house and the man will go and you get money and buy the houses!

Well, if you use the freeze time cheat, all the houses and shops will be up for sale. If you don't know the cheat, I'll tell you what to do. Go up to the Bowerstone South gates just so they open but you don't go in. Use "Beserk" and "Assassin Rush" to run in. While the map is loading, hold the analog stick down, so right when you enter, you run right back out. Time should be frozen. Then if you go in any town, everything is for sale. Hope I helped!

Just marry the wives in those houses and you will get the house.

Get all the people in town to follow you out of town and leave them in another area when you go back into town their houses and shops should be up for sell. If you want to use the freeze time cheat all you need is Assassin Rush, you don't have to have Beserk.
Assassin Rush through any gate leading to another area and in the middle of the load screen hold you left thumbstick back so that you will run back through the gate. If the gate is too narrow or at an angle from the path and you run into the gate before coming back through it, it won't work.
Freezing time won't always work for all buildings and if there are no building up for sell, leave the area with time still frozen and return to that area. To unfreeze time, open a chest or dig in the ground.