When a subject is "up for discussion", it means that you will discuss the topic before a decision is made about it. For example, you could discuss whether person A or person B will win a debate based on how well or poor their argument was delivered. "Personally I thought person A won the debate, but as the debate was very close, it's still up for discussion. Let's check with the audience".
When a house is "up for sale", it means that the sellers of the house are looking for buyers - the house is being sold. "After I have added $20,000 dollars of renovations to the kitchen and living room, I'm putting my house up for sale."