As a former assistant manager at a few retail stores, salespeople are supposed to give an informed suggestion on products. In retail like elsewhere, there's a running joke... "Opinions are like elbows ( actually we refer to another singular piece of anatomy, but being respectful ), everyone has at least one." This isn't to say the opinion is correct, as it's just as likely you could ask another salesperson the same question and get a different response. That's why salespeople are asked to give suggestions, indicate that if they don't know a true answer to a customer's question, say they will ask someone who does. THAT's what keeps customers coming back, knowing the salespeople they are talking to are honest, and not seemingly working on a commission.
You NEVER discourage a customer from buying something, if they are intent on buying it. You may suggest that it may not fit their needs based on what they are looking for based on what they told you. The customer came and saw a copy of CoD Ghosts for the xBox One they are expecting to get. Unless that salesperson has spent time playing the game themselves for a period of time, it wasn't their place to say the game is good or bad on the One. That's fanboyism talking, and cost the store a sale. Something anyone will tell you, the store doesn't want.
I never said the salesperson lied. I questioned whether the salesperson had the experience to make such a statement. You're seemingly working under the assumption that all Gamestop employees have played games on both the PS4 & xBox One. My understanding is only managers have had such hands on time early. The salesperson never says they played the game, it's an inference being made by you that they have. At this point, there isn't a large number of people who have had much time with Ghosts on the One, that I'd trust just anyone's opinion.
What my salespeople would have said at worst, is "you may want to wait". "If you want the game now, don't open it yet, get the reviews, decide. If you don't open it, you can return it for a full refund." Gamestop's will also let you return an opened game within a week, if you haven't abused that service. Bottom line, customers decide if they buy a product, not the salesperson.