should i give my imei number to a potential online buyer on my old iphone 8 i have for sale?

If you’ve not already done so …

… Please DO make sure that your “for sale” device is throughly “scrubbed clean” of your own personal info AND is not activation-locked by following this checklist:

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

There is no problem in giving your EMEI number.

The EMEI number is not tied to your personal information, but is simply the number that the network providers use to identify which phones use their network with their SIM card. The buyer will likely check with the authorities if your device has been reported stolen.