If you did a good deed, leave it at that. There's no room for resentment in a good deed. If you are frustrated enough to start a thread about your "payback", then you haven't been through much in life. Anytime you "gift" something (especially on the internet), it should be given with the best intentions and that's it.
That said, if it was an agreed upon "trade", then that sucks, and it ain't right. Who is this prominent member? Have they posted in awhile? Hopefully nothing bad happened to them to prevent them from responding. I would choose to believe the best before accusing them of being a weasel or scamming. Especially if they have been a member for awhile. Let it go, move on. You did a good thing and that's all that matters.
edit: For the record, about a year ago, Husker sent me a bunch of his awesome sauces, and I have yet to repay him with my Scovie Powder. I feel awful but he knows that when I finally get around to making it, he is #1 on the list. It was not an agreed upon "trade", he just sent them to me and I want to repay him. That's how it should be. Gifts. All around.