Is it legal for me to sell software that I have bought and used?
In some circumstances, yes, as long as you follow the terms of transfer outlined in your license agreement. You may not rent or lease the software, but you may transfer your rights under the End User License Agreement (EULA) on a permanent basis provided you transfer all copies of the software and all written materials, including the original license agreement and the Certificate of Authenticity (COA) where applicable. For any valid transfer, the software recipient must agree to the terms of the EULA. Any transfer must include the most recent product upgrade as well as any prior versions that you have.
Prior to transferring your software, you must remove all copies of the product from your computer, including your portable computer, in instances where a second copy is allowed.
If you want to transfer software that was licensed under a multiple license pack, you must transfer all the product copies to a single new owner. Microsoft License Paks cannot be broken up.
Many Microsoft products distributed by original equipment manufacturers (OEMs) may not be transferred under any circumstances. Check the terms of the EULA accompanying the OEM products to determine transferability.