I almost purchased a used 300mm f4L that had a hell lot of fungus in it. Luckily, I had a chance to check the lens before I handed over the cash.
I don't buy on ebay anymore because my earlier copy of 50mm f/1.4 came in bad shape and they did nothing to sort out matters.
Anyhow, I live in a humid environment where it rains for more than 7 months n a year. I have got a dry cabinet for my gear. No problems till this day