is it safe to share computer serial number to potential buyers?

No. People ask because they want to enter the serial number on Apple's site to see if it turns up as a legitimate number. But crooks can also use the number to claim it's their phone, it was stolen and they want the IMEI blacklisted. Or, to put in an insurance or warranty claim for a device they never owned.

Is it common? No, but it happens. More in depth here:

https://www.guidingtech.com/66540/serial-number-privacy/

what buyers want is assurance that it's not stolen.

There are better ways (such as liberal returns, and 'satisfaction guaranteed' and a history of good transactions on the barter site that can help them feel better.