The flag "Want it" is automatically deleted after 60 days. This is to keep the "Requestlist" of a collector small and only new possible trades are listed. This flag is also deleted when you have got the label in the mean time. That means when you click "I have it" than the "Want it" flag of this label is reset.

When you have got a label from your trade partner, than click "I have it". That way this labels is removed from the "Requestlist" of your trade partner and he does know not to send it to you again.

And - may be you already know it - when you have got a label, it is also removed from all trade lists. This saves your time, when you pick labels from a trade list the next time, because it does contain only labels you don't have so far.