If it's part of a quest, you need to look underneath one of the plant pots outside for a key to the house, make sure that the witch doesn't spot you once your in the garden.
You can find tomatoes at the witches house in Draynor Village.
no you can not sell your house on runescape.
No, Your RuneScape House is based on your Construction Level.
yes it is part of witches potion
use the games website runehq.com to help with quests
You can find witches in the following places:~ Draynor Village, house not far from bank.~ Port Sarim, Jewellery Shop.~ Rimmington
look on this quest guide: http://runescape.wikia.com/wiki/Witches_Potion
Camelot
By getting a life.
A witches house that has a cauldron
In RuneScape, the witches do not wear capes. If you were to put on clothes to look like a witch, you may put on a cape. You don't have to, but it is your choice.
Go to the place with the house icon in varrock.