The Go Daddy domain buy service is a huge scam!!! DO NOT USE THIS SERVICE! You pay for the service and then you have to basically put your cards on the table - there is no negotiation because if they don't get theamount they are looking for they just say the negotiation was unsuccessful, It is a huge SCAM!!! Furthermore they dont even contact you to let you know you were unsuccessful, they just close the case you dont get an opportunity to change your so called bid!!!!!
Also, most of the time GoDaddy has the domain already registered so they are looking for top dollar for the domain name so if you aren't prepared to pay big bucks they just ignore you.
I have never purchased the Domain Buy service as a buyer, but they have contacted me as a potential seller. It has the benefits of being seen as a serious offer and if an agreement is made you have a 3rd party making sure the transaction between the buyer and seller is on the up and up.
The drawbacks for the buyer are they may not be able to get into contact with the current owner or they may respond back there's no interest in selling. But if a potential seller does indicate interest in selling there is negotiation on the price. They can accept an offer, counter offer, decline... but keep in mind there is no obligation for a given domain owner to participate unless they make a deal and they aren't required to give a reason.
So it is a great service if a deal is made, but not so good if it doesn't happen. C'est la vie.