There are several ways:
Now with the 6/1/10 Patch update, you can access the house from going north from the cooks guild, passing 2 guards.
Other ways include going to edgevile ans walking east (across the bridge) and before you hit the wall of the GE you can head south.
no you can not sell your house on runescape.
No, Your RuneScape House is based on your Construction Level.
Camelot
Go to the place with the house icon in varrock.
in the volcano at michaels house
Yes.
open the door
Yes.
you build on your house
No.
yes
There is nothing illegal about playing RuneScape.