Need advise with buying a strop

Hello everyone, I'm about to buy a second hand strop, but I wanted to double check.
Under inspection it seems fine, is not cracked or hasn't been cut or anything like that.
However there're some "spots" on it. It seems to me those are moist marks, but might be something else.

I attached a pic. of the strop (for which I'm been asked about US$12) I know is cheap, but if is not good, is money wasted.

Could you please guys give me an idea if is worth buying or not?

Thanks,
Pablo.

Basically:
Rub your hand on it, Not Smooth=BAD
Look at it, Cupped In The Middle=BAD