I was thinking of maybe starting to grow my own mangroves since theres a canal behind my house that has brackish water. I was just wondering if anyone would be interested in buying, and if so how much would you pay, not including shipping?

Simple,
They would definitly be a sump only plant in an octo tank as they are hard to keep in place just in a sump. I have had them in my sumps and they last for a long time even without the proper care (the tops need to be above water and rinsed with fresh water on a regular basis) but they don't really seem to do much for the water and I finally eliminated them. To get a good idea on expected pricing, look on eBay (where I picked up some of mine and could also be a market for you if you think the going price is acceptable).