The irony of the law is, it's perfectly legal for you to buy tickets from a tout, but it is illegal for him to sell them to you! You will usually find that tickets are genuine - have a good look at them before you buy (take a torch, it's dark ourtside most venues) and see if the paper is good quality - if any embossing is raised, the usual ways tickets are printed to avoid conterfeiting, which is a different thing althogether. Chances are you'll be OK, but leave it unitil the last possible moment before you buy, and haggle if you feel up to it - by then the touts will be keen to get something rather than nothing, so don't be too keen to accept the first offer you get. Enjoy the gig.