I also walked to the Sumas border crossing and talked to them on a slow day. They were great and helpful.
Yes, if you try to bring something across that is not allowed, you can get in big trouble. More so if you fail to declare or hide it from them etc.
It is important that you have all paperwork in order to make it easy for them. They can not tell this from that, so they sometimes just say it is not allowed when it is. So pics and info helps them. I have not brought lots across, but what I did the one time, went fairly smoothly.
http://www.aquacorals.com/CanadaImportLaw.htm