Pretty sure it can only be done thru the peace arch and the truck crossing. For simplicity it's better to deal with the guys that do it day in and day out. No need to start off the trip on a bad experience and a possible major time delay.
Truck crossing, NOT peace arch. You don't have to do anything on the Canadian side (unless your buyer's broker is there). You have to go on the TRUCK side, with all the commercial vehicles. Even though you are a private citizen, you are acting as the importer into the USA and are thus considered crossing for commercial reasons. Be prepared that agents on the commercial side are likely to be difficult, and perhaps "rude" to you. You won't know the system, and they won't be very helpful with you on it.
Other than that...it's a lot of fun! </sarcasm>
-Dave
(did it within the last 12mo)