I've seen that a few times. By that I mean once, from the user which is actually buy.com
I just went through the hurdles to ask a question and it is letting me do it. I always just click the 'no this didn't answer whatever'. Want me to ask for you?
Thanks, but I just bought one off mmmty. (thanks mmmty!)
I know you click through the "Find answers from" crap, then click "No, I want to contact the seller." The problem is that many times it then takes you to a page that says, "We're sorry we couldn't find an answer for you. Unfortunately, this seller is not able to respond to your question. We suggest reviewing the item again to see if your answer is in the seller's listing." After that, you're screwed. I've seen that message at least 20 times - usually when the seller sells only to the US. I think it must be a default setting to prevent non-US buyers from asking questions from a US-only seller.
It's not on all listings though. I just asked another US-only seller if he can sell to Canada (for Belkin PS/2 to USB), so it's possible to contact some of them.
Edit: It might look like I'm constantly harassing US sellers, but most of sellers I've talked to didn't even know they neglected to include Canada (typical - Americans always forget Canada exists) and they change the listing to include Canada.