Can I find my IMEI number using only my serial number

You can’t find out the IMEI from the serial number. I hope you bought it from someone you know or someone who will stand behind it and that you can contact after the purchase. See this user tip—>The All Too Common SAD Reality of Buying … - Apple Community

You can find the warranty status of the phone from this link—>Check Your Service and Support Coverage - Apple Support

And if you are in the US you can see if the phone was reported stolen—>CTIA Stolen Phone Checker

Why don’t you ask the seller what the IMEI is?

The number can be found on the device itself.

Look under Settings.