I found a great Paypal fee calculator that you could use to estimate your fees. Just enter the price of the item you're selling, and it will give you the price you should list it for: http://thefeecalculator.com/ There are other paypal calculators that you could find online (check the updated rates).
"A humble and friendly message"
If Paypal is the only payment option you're providing, asking for additional merchant fee is not professional. Just add the fee to your sale price before listing it under classified. If your buyer insist on Paypal despite having other payment options, it will then be a common practice to discuss additional fees.
Thanks