I bought this cactus recently from farmer's market and it is becoming brown from the bottom. I have stopped watering it but is there anything else I can do to save it. It is kept outside in spot that gets about 5-6 hrs of sun. Should I repot this? I have also removed the weeds shown in the picture.
Looking at your photos, it does look like a disease rather than older scar tissue from age and sun. It looks like either a bacterial or a fungal rot may be invading your cactus. The health of the weeds around the cactus might be a sign that the cactus has been watered too much. If it is indeed rot, then the brown tissue will be softer (not hard and firm), and the roots are probably infected (no good).
That being said, it may be possible to save the plant by slicing it above the damaged tissue and rooting the "cutting". This can be somewhat tricky, but columnar cactuses, such as yours, can and will root from such healthy cuttings. I have include some links that might be of further help. You can do some more research around these topics if you need to.