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Make sure that your trade offer URL that is located in your scrap.tf settings is correct.
Check if the items that you are trying to sell are tradable. (metal/items)
Make sure that you haven't changed your password recently, logged in from a new browser, cleared cache and/or cookies or recently activated Steam Guard. If you have done anything of that then you will have to wait until the time limit is over so you are able to trade again.
Your backpack might be full or might not have enough space for your new item(s).
Your trade offer URL might not be working properly or you didn't set it properly.
Trade Offer
Auto-Sell Hats
Automatically sell up to 100 of your eligible hats at once.
Don't worry: we won't take special items, including painted, low craft numbers, special levels, spells, killstreaks, etc.
Autosell Cards
Automatically sell up to 1,000 of your eligible cards at once.