That's most likely because the bot's CSGO inventory is full, as said inventory can only have 1,000 items at once. You can verify so by checking the bot's CSGO inventory in Steam.
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.