I live in Canada (and set it as so), but can receive cards in Canada or USA. I noticed when I'm looking at sellers, it says I can't buy from them because they don't ship to Canada.

Is there a way to set it so I can buy those cards?

Sellers select which countries they are willing to ship to, not Deckbox.

The most common reason sellers are not willing to ship internationally is the cost of tracking the packages. For example, the cost to send a bubble mailer with a reasonable number of cards (3 oz. or less) from the US to Canada with tracking would be roughly $20-35. In contrast, First-Class International Mail of the same weight will cost less than $3, but does not provide tracking. The only alternative is Registered Mail which requires a signature for the package to be delivered. This costs $13.65 which is (I believe) in addition to the ~$3 for First-Class International Mail.

(Everything in USD)

Yeah, I understand that.

My point is that I can receive in either Canada or US. But, for some sellers, they won't ship to Canada so I can't purchase from them. I'd like to purchase from US sellers to ship to a US address.

That's all.

Ah sorry, I missed the "...but can receive cards in Canada or USA" part. This definitely seems like a unique situation. Hopefully Sebi will respond tomorrow morning with some sort of fix for you.

Yep, right now it is not possible. You can get around that (albeit a bit of a hassle) by setting your "main country" to U.S.A temporarily until you place the order, and when you place the order select a US address.