HI Anne
Personally, I would not get involved until the listing is either terminated or expires.
But, it is possible to both be involved at the same time if the seller tells the listing agent to remove the property from MLS. The seller would still be responsible for paying a commission to the listing agent (hopefully not a full 6%), so that will really cut in to your profit and/or make it not advantageous.
Obviously, this agent has not done anything to help sell the house so it is a shame that they would get paid for YOUR efforts.
When does the listing expire?