No pics and no scientific names so, I guess we have to break out the crystal ball:
By 'marble lung', do you mean the african Protopterus aethiopicus? If so, $40 is a good deal for an 8 incher. The spotted lungfish can be either the african Protopterus annectens or the south american Lepidosiren paradoxa. And, $40 for either one, as a 15 incher, is also a good deal. Each of the african lungs would make a great display in a "one of" tank since they can't be trusted with other fish including their own kind. The south american lung would do OK in a community tank provided its tankmates were too big to become a meal.
The P. aethiopicus grows to 78". P. annectens to 40". And L. paradoxa to 49".