Just from the single scene, it seems very clear that Usopp is being manipulated and scammed by the pirate artifact shop. This sets up a very solid critique of capitalism, wherein profit motive is the only driving force behind any business model. The salesperson appears to be using deceptive language, and possibly outright lies, to convince Usopp that these items are of supreme value, both practically & sentimentally, yet it is much more likely that they are cheap reproductions that will soon fail and are not worth nearly the price this shop will charge for them. Usopp, being inexperienced in the ways of the world outside his village, has no reason to distrust the shopkeeper, and so is easily identified as an easy mark for a con.
The barman in the Gold Roger Bar is maintaining the oral tradition of storytelling, passing on unique perspectives and facts that cannot be learned elsewhere. Much of this is his own firsthand experience but some is also being retold from the stories that people passing through his bar have shared with him. When we see that modern people are not necessarily interested in his stories anymore, this is a strong parallel for the real world in which recorded media, music, movies, television, etc, have replaced a lot of genuine connection and interactions between people.
Getting some more insight into the execution of Gold Roger is a powerful look at stoic enlightenment. This is a person who achieved some of the greatest dreams on the planet, wealth, fame, & power, and knows both what it takes to gain these things as well as how it feels to hold them. However he was captured, it did not cause him to lose his positive outlook and calm attitude. It is very likely that this outlook & attitude was a major contributor to his being able to become the king of the pirates, and he did not lose them even upon his execution. We see much of these same traits in Luffy.
Last note, the execution platform is much larger in the memory sequence than it is when Luffy finally finds it. This is because the memory is told from the perspective of Admiral Smoker, who was a small child at the time, and it seemed much bigger to him then. All of our memories are typically tainted by our own perspectives & biases, and it is important to recognize these details and how they affect our own narrative, self-image, & decision making process.