I personally don't mind it unless said person is pressuring me/insistent on me reselling. But if they're asking for a heads up if a character goes for sale, I really honestly don't mind at all. I do have it specified on my profile that none of my charas for sale but also that people could ask if they are actually interested for character sales.
I haven't gotten anyone actually rude about it to me, so that's probably why I'm 100% okay with it. I'd imagine if someone became snippy with me tho I'd like it a lot less. But otherwise, it helps, at least, if I'm in a hurry to sell.