dust in the lens is rarely, if ever a problem, unless it is big dust balls, which will reduce contrast, but I have a few different lenese that have got dust in them over the years and has never cause a problem..but it is a very good negotiation point

I purchase a 300mm f/2.8 a couple of years ago and got a real good deal because of a 2mm scratch on the outside edge of the front element and it has been a great lens with no ill effects because of the scratch

Dave