A collection PIN is a unique number that will be generated for each Gumtree purchase. This unique number is the way that you can be sure your money is kept safe until you hand over the item to your buyer. 

When you meet with the buyer, before you release the item to them, the buyer will share the Collection PIN number with you. The you should then enter this number on your Gumtree app in order to conclude the sale and get your funds released. 

You should ONLY release the item to the buyer once they share the PIN, and you have entered it on the app to ensure the number is valid. Do not share this PIN with the seller until you are happy that you have your purchase and are ready to finalise the sale.