Might as well get it resolved now, because when you sell it, it could come back to haunt you later on. If the plumbing is backing up, then I would have the system checked. If it is a septic system, then the tank might need to be vented. You can't vent in the cellar, but need to vent at the tank end. Air has to escape from the tank if the water is to go in. Usually, the tank vents through the pipe that goes through the roof. Have you checked to see if there is a clog there? Could be as simple as a birds nest causing the problem. That pipe should be one straight line to the basement pipe, and all the fixtures should be branched into it from the sides. I have seen them get clogged and not vent properly. I had to put a mushroom cap on my tank at the old house, because the ground wouldn't let the tank vent properly.