Should I install a bathroom vent fan?

[Michael Wei]

We're working on a flip (well, not exactly, it was a long term hold that we're about to sell), and I just noticed there's no bathroom fan in place in either of the two bathrooms. 

Normally, I'd install them, but the "attic" (so. california houses...) is rather low on this property and it'd be a bit of work to properly vent to outside. 

Wondering if anyone knows if buyers will care about it, and if I should just install now. Both bathrooms DO have windows, but we found that one of the bathrooms had mold. As a reference, the house is worth about ~500K.