No need to panic if the ring you bought to sell doesn't have a size tag!  It is actually super simple to figure out the exact size.  Here's all you need:

This awesome, super cheap tool is called a "Ring Sizer Gauge" and it allows you to figure out the size of the ring mega-fast!  Best of all, you can buy this entire tool for $4 - $8 on Amazon. 

IMPORTANT TIPS:

1. If you are a USA seller, selling USA products, make sure the sizer of your choice is in USA sizes.  Some sizers are made in UK sizes or other countries sizes.  

2. I would encourage you to read the reviews and make sure the one you are buying has lots of happy customers.  There will always be some people who complain that the product isn't good, but if the vast majority are happy you can assume the sizer is legit. 

3. When the sizer arrives, test it compared to a ring that you know the size of.