Goblin society outside room 9 has never been particularly commerce minded, and inside the dungeon currency is an abstract concept, so barter and trade are the norm. The basic unit of trade is the 'useful thing', and on occasion spare or rare useful things, and plausibly unique useful things.
Some useful things might have a material characteristic e.g. useful thing (wood), or useful thing (iron), which mat be relevant in making items out of useful things - see below.
Typical useful things might include a short length of rope, a solid (ish) plank of wood, a shaped piece of stone, half a dozen nails, a broken bottle, a slightly mouldy melon, a door handle, several pages from a book with illustrations, a shard of a mirror, a bucket lid etc etc
For ease of play a useful thing, no matter what material, has the trading power of a gold piece in the rulebook. It is however more normal for goblins to make their own items, weapons and armor - well kind of weapons of armor out of useful things.
Making things
Gobline like to make items out of useful things, and at worst they will be armed with improvised weapons and (most of) armor. With an interesting story, or description from a player, they can make almost any item from the main rulebook - albeit it with a -4 attack / skill modified ala the Improvised Weapons rules on p144.
Goblins with appropriate skills and/or feats can make almost decent weapons, (sort of) armor, traps and tools - although again the story / description is important. Some special items might require specific useful things to make similar to the crafting of magic items p538 - 553. The idea of goblins actually finding magic items or useful things (magic) and then making other items and devices out of them is kind of terrifying.
Image above found here; Wizard of the Coast - ten things you might not know about goblins.