Not sure if the info in this post is general knowledge. I suspect that it is not.
I was pretty happy when I got my first truesilver line. Seth's Graphite Fishing Pole worked just that little bit smoother. Then I got my second. What to do. I put it in my bank in case I ended up with a better rod at sometime in the future. Then I got my third. I destroyed it because I didn't have the bank space.
That was a big mistake. I've subsequently found that you can use the truesilver line on a Big Iron Fishing Pole and it doesn't make it BOP. I bought a rod for 28g and sold it at AH with the new line for 120g. I got another truesilver line and put it on a guildies BIFP. I confirmed that you can not do the upgrade in the "not to be traded" box. You have to have the rod in your possession.
So don't destroy those extra truesilver lines. Upgrade a BIFP for someone else and make some decent cash.
Tobruk
Warlock
Khaz Modan