I think it's O.K. to have the listing agent buy the house form the seller, as long as the two agreed on the terms and conditions. If the seller is happy with what he is getting, then it should be O.K. However, if the seller has any doubt, then it is best not to pursue the sale to avoid further complication. But again, if both sides are happy, then that's a deal made.
On the second issue, I wonder why the listing agent wants the seller to carry the loan. Was that something the seller already offered to any buyers, or is it because the listing agent can not get a regular loan otherwise? If the seller agrees with this (along with the interest rate, the payment plan, etc) of the loan, it should be drawn up by an attorney to make sure the seller is protected should the listing agent default on the loan for whatever reason (same as if it's purchased by any other buyer asking the seller to carry back).
Basically if all is out in the open, agreed upon, with real estate attorney advise. then I would think it's O.K.