selling old iPhone 8- need to remove sim card

There are a bunch of ways. I've received a SIM eject tool (basically just a pin that looks like it's attached to a closed tuning fork) packaged with a SIM card before. However, Apple's instructions mention using either a SIM eject tool or a paper clip, and their demo images all show paper clips being used. Just be careful as paper clips aren't as strong and can bend and/or damage the finish if it slips.

Remove or switch the SIM card in your iPhone - Apple Support

The simplest tool to use for this would be a paperclip.

Ref: Remove or switch the SIM card in your iPhone - Apple Support