Everyone has different thoughts, but whenever I see an ad that says "paypal add 3%", it leaves a really bad taste in my mouth. I would not buy anything from these sellers.
Sellers - wake up! This is not something that will help you sell your stuff. Just list your pricing with the expectation that YOU will cover paypal costs. This is just like any other vendor that will cover credit card fees if you choose to use a credit card. No penalty for using a credit card or paypal for the consumer.