If the person asking is a member here and has been active in the forums and contributes with post regularly you get to know some things about them. If any of those people ask about trades or plants for postage, I am happy to respond to them. Maybe I can or maybe I can't, maybe I will or maybe I won't, but I don't get offended if they ask.
I do have to take into consideration how much time and energy I will have to spend on the project, and if I actually have enough of the seeds or plants to share. Often I will be saving the plants for trades (the best way to increase you selection at affordable cost).
I have found that sometimes when people are asking for free seeds or plants they have not given any thought to how much trouble it is to collect and save seeds, and especially how much time, and effort it takes to dig and pack and ship plants.
I have sent plants and seeds to several members here on the forum, and believe me I enjoyed communicating and sharing with them all.
Now if it is just someone who just registered, has not taken the time to let people get to know them. If they have not participated in the forums, have submitted no photos, etc. That is a whole different story.