reset to factory settings, i want to reset my imac for resale, running high sierra

Adding to the excellent advice provided by rkaufmann87: What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

Unless that iMac shipped with operating system installation media on DVD discs, you must absolutely, positively create a "bootable USB installer" before erasing that Mac. There is no other practicable way of reinstalling High Sierra or anything earlier.

Do it now: Create a bootable installer - Apple Support. After you erase it, it's too late.

Please Google for Apples link that explains “What to do before you sell, give away or donate a Mac.” That will tell you precisely what to do.