This is a relatively new precaution to protect both sellers and buyers. It's meant to b moreso for buyers though. I had this happen to me recently as a buyer, and the seller was V. annoyed that the funds were held -- unfortunately, there's nothing you can do about it! They do it for a number of reasons - either you sell an item which has a high number of replia/fakes around, or the sale was over a certain amount, or your feedback isn't 100% or you're new, and haven't been a member for over 90 days.
It will be lifted either a) when the buyer leaves you positive feedback or b) 21 days have passed and there has been no complaint.
Hope that helps you -- but note from a buyers persepctive, the funds are already out of his/her account, and they won't know the funds are on hold, so bare that in mind that they have no control over it!
Sarah