I must admit unless someone has something to trade I usually ask a small payment, especially if they want several discs.
The reasons are simple, firstly I get quite a large number of requests for shows so over time I would be quite a bit out of pocket on discs and postage and second many of the shows I have I've had to buy in the first place, often at over inflated prices!
I really do prefer to trade but if that's not possible I don't think a very small payment is unreasonable to cover time, discs and postage and people seem happy with this. I occasionally do a freebie but couldn't do it all the time. I pride myself on offering a good service and great value to those people who do buy or trade with me.