A more or less complete guide to trading by ya_
GD is a difficult game and it’s complexity can sometimes intimidate new players. Therefore, I advocate a friendly trading environment. I trade my items for whatever you can pay with as long as I can find use for it. I try to avoid giving out items for free because it contradicts the “philosophy”: you have to put in some effort to get what you want or it doesn’t make sense - you could as well GDStash everything.
As for the types of mats, advanced players usually want iron bits, scavenged platings and scrap for the crafting of Stoneplate Grieves. Scavenged platings can be vendor-farmed by visiting Isaiah Reddan in Homestead, Morris Burton upstairs from Isaiah and then Hyram in Steelcap, Malmouth. Repeating this will net you scavenged platings but also lots of scrap, aether crystals, shards and clusters, other common and rare mats, potions, low and mid level relic blueprints and others. Vendors’ stock is reset after visiting two other vendors but you have to buy something or they won’t be counted as visited. Farming or crafting dynamite and dismantling epics and legendaries gives a lot of scrap. Money and scavenged platings come along as you level your chars - just don’t get lazy filtering out yellows or skipping chests. Other desirables vary from patch to patch. These days in short supply are (IMO in this order):
battered shells/frozen hearts (mutually convertible so basically the same item; farm in Royal Hive),
chthonic seals (low drop rates make them a real pain to farm; Darkvale),
ancient armors (totally random very rare drop),
ugdenblooms (easy farm in lower Ugdenbog some or all the way to Ancient Grove)
Pre-AoM rare mats (come along as you farm bosses, also dynamite stuff)
Mutated scales (sometimes some people might be out)
However, the most desirable things are items themselves. Do not vendor or blow up mythical legendaries or even low level ones. If you run out of stash space create a mule character. Myself, I tend to keep 3-4 copies of an item before I start cleaning up. Also, many advanced players have their favorite chars whom they minmax (collect gear with best rolls). So try to keep a list of your items so that a potential trade partner can look through it and tell you which items’ rolls he wants to check out. How do you know if an item has a good roll? Go grimtools.com -> item database -> settings -> show min/max values and search for your item.