What they said! Mold can eat through, and effectively destroy, paint. But it will not affect metal; only rust (corrosion, oxidation) does that. I've seen things that looked like they were dug out after being buried in primordial slime, turn into gleaming new treasures, with little more than a good cleaning.
At that price, STOP READING AND GO GET THOSE! They are worth it, mold or no mold. Only if they are mis-shapen lumps of crumbly corrosion --that is, nothing worth salvaging-- would I pass on any early Marantz that was cheap. The parts that are restorable will always have value, and if the main issue is mold, it is very likely that the things will be restore-able, in which case they may have 'serious' value.
One question I would have is whether the mold that eaten into the potting material and wire insulation in the transformers, and thus possibly "killing" them. That could be a problem, but it isn't very likely, and even if that were the case, the units would still be worth that asking price.
