They play a huge part in the adventure, meaning, they play a huge part in the Basic Adventure, the Lost Mine of Phandelver, set near Phandalin. Events of that adventure shaped the region, unless the characters haven't ran through it, in which case, forget what I said!! LOL!!
My players will be at least lvl 10, if not higher, by the time they finish with LMOP. :P
Phandalin is a mining town, so if you wanted to sell a bunch of scrap metal, that's probably where you'd go. I don't see any economic reason why your chars wouldn't be able to sell their scrap. They (the phandelonians) should have lines of trade setup with neverwinter and waterdeep. If your characters failed to clear out the redbrands, and/or report the head of the miner's guild as a zheterim agent, slidar might be dead when you return and the redbrand problem could be a problem again. I imagine that the result of that would be a high tax rate, and people looking for bribes/to skim your player's profits.
Raw ingots are going to be worth about 20GP each
Pure adamantine barstock is going to be worth about 100gp/lb
Adamantine coated/forged objects can be sold for 50% by raw.
If the players bring A LOT, a lot of material, give them the option of becoming a licensed mining interest, which would allow them to rent space on a wagon, hire wagoners, etc. There would be a number of upfront fees, but you'd only be taxed at 5% instead of 20% (25%/40% if the zhetarim are in charge)...