Make sure you are buying from a reputable Retailer/Vendor and that the Motherboard has at least some type of Warranty on it from the Retailer/Vendor.
Also once you get the used Motherboard, open a Support Ticket with the manufacturer of the Motherboard and give them the Serial Number to find out how old the Motherboard is.
Also find out if you need to update the BIOS to make whatever processor you are planning on installing to be recognized and boot up.
You can go to the Motherboard's Support download page to find out if there are any newer BIOS versions to install.
When you get the Motherboard I would do a physical check of the entire PCB to make sure there are no unusual spots or marks or broken electronic components and that the Motherboard has it native CPU Cooler brackets installed and not missing.
Also if there is no Motherboard manual, download it from the Manufacturer's Support site and see which screw holes you need to use to install the Motherboard in your Computer case.
All of the above is because you are buying a Used hardware so it is best to try and get as much information about it and make sure everything looks good and you have a least some sort of Warranty on it.