Agreed, when it comes to this sport you should never for any reason be forced to buy something you didn't want. Not in Canada at least.
I will say the seller is still in a spot where he can make it up to the buyer with a reduced price or some add on's or even a good price on another item, if it was me and the seller was willing to do that, then I'd alright with it.
As far as neg feedback, you're right, that's just black mail. I know personally when I'm looking at adds, if I see an add that threatens the buyer with neg feed back, I'm less likely to contact them. I tend to protect my trader rating because having a perfect trader rating is the best way to let a potential buyer know that they aren't going to get screwed with. But this works both ways.