Erased hard drive Reinstalled OS but automatically connected to network after set up

please see this article: What to do before you sell, give away, or trade in your Mac - Apple Support

as per step 7 of the above article:

After macOS installation is complete, the Mac restarts to a setup assistant that asks you to choose a country or region. To leave the Mac in an out-of-box state, don't continue setup. Instead, press Command-Q to shut down the Mac. When the new owner turns on the Mac, the setup assistant guides them through the setup process.

you should let the new owner run setup assistant after you remove your self from the mac.

Sorry, I somehow missed the fact that you’re selling the Mac. In that case, you will want to try the reset from the beginning, following this article step by step: https://support.apple.com/en-us/HT201065

And as JeffreyTheFrog said, you shouldn’t go through setup assistant after erasing the disk and installing a fresh copy of macOS. That can be left for the next owner to do.

I believe certain wifi/network information is stored in your Mac’s NVRAM (non-volatile random-access memory).

Reset NVRAM: Shut down your Mac, then turn it on and immediately press and hold these four keys together: Option, Command, P, and R. You can release the keys after about 20 seconds, during which your Mac might appear to restart.

if you couldn't sign out because you were not signed in, then yes, you are signed out. if you have already run setup assistant, i would recommend starting the erase process over and your info will no longer be on the mac. you don't need to sign out if you're not signed in. then when the startup assistant starts, press "command+Q" to shut down, and you will be selling a "blank canvas".

If I was you, I would do it over again - this time follow the instructions and after you boot into recovery, erase again and download the OS. When the download is finished, just shut down (use the power button to turn it off) the Mac and leave it at that. That is how you need to do it nowadays.

If you go any further, you will automatically have the Mac connect to things it shouldn't be connecting to. If you turn it off, the buyer will get the same screen as you do when it is new and they can set it up using their own Apple ID, network, etc.

Hold Command+Option+P+R during startup and hold down until the Apple Logo comes on twice, or for 20-25 seconds. It might reset Network Preferences