Copyright is the key thing to look for if considering purchasing old photographs of buses. Photos can be easily reproduced if one is not in possession of the original negative. So while some traders may claim that the copyright is transferred to the buyer upon purchase; an unscrupulous trader may hang onto the negative in order to reproduce and resell the same photo at another event/fair. If two people decide to upload the same photo to Flickr or whatever, then this has the potential to cause problems as each could claim that their copyright has been breached.
I don't have a problem with people hoarding photographs to sell at a later date. Some people share their work while others don't. If people are willing to pay for photographs of buses then so be it - it's a free market and it's not worth getting upset about. But if you are going to purchase an old photo, try and make sure it's the negative as the likelyhood of it being copied and resold is less than a print (if that makes sense).
RE: Anything and Everything