I realize I have no control over resale of my novels, and I think selling used books is a kind of free publicity, so I have no problem with it.
I don't do booksignings anymore, but I do give away a lot of signed books, which I also ship to the recipients for free. Occasionally I'll include a personal note, especially when I think someone needs encouragement.
Sometimes people sell those signed books, and while I don't care for it, I ignore it. But when my readers tell me about stuff like this, it makes me seriously contemplate never signing another book or writing another letter again.
I know, anything I write is potentially a money maker for someone, but selling my personal correspondence is tacky. Please don't do it.
Update: I checked my records (yes, I keep records on this kind of thing) and this item was a book I sent to the son of one of my editors, at the editor's request. I was told this individual was feeling discouraged and was asked to send him an entire set of my SF books. Which I did, stupid me, so more of those books may be showing up on eBay.
What truly burns about this situation is that I assumed this person was trustworthy and put my personal contact information, including my address, on that particular letter. None of the info is correct anymore, in case it comes up for sale again and you stalkers out there are tempted, but still. Last time I do something like that for an editor.