What are you meaning by old? Are you talking about vintage manual focus lenses or lenses that are just a few years old?
I have plenty of vintage lenses bought from Ebay for use on my Nikon DSLR and an Olympus micro 4/3). I have never paid much (usually the Australian dollar equivalent of around 10 or 20 pounds, occasionally more, several for less - I just scored a Rokinon 80-200 and a teleconverter, both for Nikon for 10 cents, which I think is about 7 pence). I have not had any problems and scored some really nice lenses at absolute bargain prices. Some have proven to be rubbish, but that it's down to the lens itself, not the seller. At the prices I usually pay it it's worth the gamble and they are all fun to play with at least once.
Mould or fungus can be a big issue with old lenses. I gather it can be cleaned if you have the skills to dismantle and reassemble the precision instrument that is a lens. Not worth having professionally done unless the lens it's really worth the cost. Small amounts probably won't affect your images, but carefully check photos in the listing, read the description and ask questions.
One little tip - keep an eye out for film SLRs being sold with one or more lenses. Whether you are interested in using the camera or not, you can often get the camera and lens/es for less than the lens/es on their own, sometimes much less. Stage but true.