Sellers should not publicly share IMEI, ESN, or serial numbers for devices. Your IMEI number is considered private information until your item is sold.
Buyers can ask sellers to check the IMEI with a carrier they'd like to use by supplying a link to the carrier's BYOD (Bring Your Own Device) carrier compatibility page.
Buyers shouldn’t post listing comments asking for the IMEI before purchase.
Sellers: If you are wondering whether it is safe to send the IMEI to a buyer -- it is safe to send the IMEI to a buyer provided it is being shared privately, and after the device has been purchased.
Buyers: If you aren't sure if the phone you’re looking at is compatible with your carrier -- you can learn more about carrier compatibility on the Swappa blog:
Unlocked iPhone Carrier Compatibility Guide
Android Phone Compatibility Guides