I am looking to buy items but it only lets me seek for the amount of items I have
For example I am looking for copper ore, but if I only have 12 in my inventory it only lets me seek for 12, would be nice if it let me seek for an unlimited amount
You sure it isn't just because copper ore only stacks to 12? You can probably list seeking 12 and then separately list seeking 12 again, although I haven't tried it.
It only lets you seek to what you have.
For example if I have 8 lightning shards (stacks to 999) it will only let me seek 8 .... even if I want to buy 200 or more of the things.
You also must already own at least one, so you can't use the seeking system for say an armor upgrade or weapon upgrade.