Unless you are a high volume seller with reduced fees then it does not matter which account you sell on.
If you are selling then again it does not matter if a buyer uses the send money > goods / services option OR you send a money request or invoice as the fees are the same.
FEES are in Paypals user agreement.
The only time fees are less or none at all is when someone uses the friends / family option to send a gift of money to someone. However that option is not to be used for selling and if you do then you lose your seller protection and risk having your paypal account limited.
Advice is voluntarily given. Kudos / Solution appreciated if I helped you.