You don’t. Most discount brokers will only do a listing with the person on title. Unless you can convince the seller to transfer title to you, it will not work.
Why would you want to list with a discount broker anyway? Read the fine print on these listing agreements too – sometimes they say no matter who sells the house (including you), you will owe them a commission.
Instead of a discount broker, put the property on craigslist, zillow, trulia, etc for maximum exposure and use signs signs signs.
If you are selling to a retail buyer, contact mortgage brokers. They already have people preapproved for financing and may have a perfect buyer for you – without paying a commission.
Put flyers out in the neighborhood to advertise the house. Neighbors may want to buy or they may know someone who does!