Even the most expert salesperson can't possibly sell you your ideal shoe if the store doesn't carry it! And no store carries every shoe. Every salesperson knows this. Unfortunately, not all customers do. Salespeople know this as well, and the lesser ones take advantage of it.
One way to tell the best and very best salespeople from the second-best: Only the best will tell you when no shoe they carry will work for you. And only the VERY best will recommend you try specific shoes they don't carry, and will refer you to competitor's stores that do (though you may need to ask).
A lazy salesperson will initially bring out only one pair of running shoes for you to try. A good salesperson will bring out 3-4 pairs, and a better one will bring out 6-7. A truly dedicated (but non-expert) salesperson will keep bringing out shoes until you either buy a pair or leave (or they run out of shoes). An expert will have a process for shoe fitting that will quickly find the best shoe in stock for your foot. But only the best and the very best will know when to stop bringing out shoes, will know when no shoe they have will work for you, and will pass on the sale.
Specialty running store owners and managers will often have this level of insight, since they have to examine and consider lots of shoes before selecting the brands and models they will carry. The typical running shoe salesperson will not.
Then there are the truly bad salespeople, the ones who will say anything to sell a pair of shoes. The worst among them may mention giving running shoes "time to break-in". This is a lie, a blatant attempt to persuade you to buy a shoe that doesn't fit. Taking this bad advice means it will be your foot that 'breaks' (blisters and bleeds), not the shoe.
Long ago, when running shoe uppers were made of cotton canvas and/or leather, the shoe shape would change very noticeably during the first several hours of use, so you would buy them allowing for this behavior. The upside was that every shoe would soon become custom-fit to the wearer's foot.
Modern running shoes use synthetic materials that are much more stable: It used to be that you discarded racing shoes if they got soaked: Now we toss our shoes in the washer!
Modern shoe designs are a much closer match to the actual shape of the foot, so a better fit 'out-of-the-box' is possible, and should be expected. The downside is that a modern shoe won't, can't, change shape much (not until it wears out), so it is more important now than it has ever been that the shoe be as perfect a fit as possible on the day you buy it.
Unfortunately, too many runners have no clue what their 'perfect' fit feels like, having never had one in their lives. Even the most expert salesperson can only do so much in the face of such ignorance. The best way forward is simply to try on and test-run lots of shoes at lots of stores, learning every step of the way.
While it is useful and instructive to listen to shoe salespeople, and it is worth your time to find the true experts, nothing can replace knowing for yourself when a shoe is best for you. When you do buy a pair of running shoes, you must trust yourself above all others, and take full responsibility for the results.