I recently gave something away for free and was pleasantly surprised to find that the recipient left me a chocolate bar when they picked the item up. I didn't expect anything in return, but appreciated the gesture nonetheless. I haven't received any free goodies yet but if I did, I would most definitely leave a little token of my appreciation (even if it's just a nice thank you note, if it's not seasonally appropriate to leave edibles), and I would send a PM afterwards to thank the person again. It never hurts to be polite!