First and foremost, I believe a transaction is a private matter. If a buyer wishes to publicly acknowledge quality of service (good or otherwise), it's his prerogative. A buyer wishing to thank that ackowledgement should do so privately. A public "thank you" is a breech of confidentiality which a seller is bound, written or otherwise, to uphold.

The most obvious route my thinking took here is regarding ISOMs. After further consideration, I believe it applies to all transactions.