Too much of a good thing
You may receive this result when sending, accepting, declining, or canceling a trade offer. Below is an explanation of what this result means in this context.
Sending this trade offer would put you over your limit. You are limited to 5 Active offers (including those requiring confirmation, but excluding those in escrow) to a single recipient, or 30 Active offers total. If you are accepting a trade offer, then your inventory for a particular game may be full.