All three tests said not elevated. I think that means the levels were safe.
Having said that, if you can SEE mold present, you can clean it with bleach. If it's growing on the paneling, it probably means you have either a high moisture content in the basement or a potential water leak behind the wall (if it's local to one place). Mold is a product of temperature and moisture. Changing the humidity will stop it from growing, but it will still need to be cleaned with a mold killer.....bleach water and clean rags can do it.
The way you can fix it is either sealing the room (if vented to the outside) and installing a dehumidifier with a drain and pumping the water collected out... Sealing is difficult because it may mean removing wall coverings (panelling, etc) to get to the foundation walls....then covering them with a moisture barrier like 6 mil plastic. Be aware that some basements have issues with radon gas as well. Sealing the basement could potentially trap radon which is also a respirtory hazard.
Another solution is to install a powered vent to the outside and then another powered vent from the upstairs to downstairs in addition to dehumidification downstairs. This forces air conditioned/climate controlled air from the upstairs down and out a controlled vent....so you're basically reversing what most houses do with a chimney effect. (air enters from below and goes into the living space and out through the ceiling/windows/chimney) If you reverse it, you can blow dryer air into your basement and change the humidity that way.
In any case, since you're selling....see what they want you to do.