is it normal for buyers to ask for serial number of my iphone? can i just give it to them?

Yes, it tells them the legitimacy of the phone. the serial number is tied to the purchase of the phone and verifies date purchased and more importantly if it's still under warranty. There is no personal information given about the phone serial.

This is just the buyer being cautious of the purchase and if it is a legitimate sale, it won't matter if they have it or not.

If you want to make buyers more comfortable, offer to exchange the unit and money at a store, either Apple or service provider. That location can verify the device is legitimate.

Here's what you can do with a serial number: https://selfsolve.apple.com/agreementWarrantyDynamic.do

It's an easy way to verify the model and warranty status.