I don't know if it is common in Turkey to ask an employee if you can work there, but I know that this would seem odd in the USA. For the most part, it is not the employee's job to handle applications and potential new hires. When I worked in retail, the default was to immediately defer to the manager when someone asked about it, or to tell them to come back when the manager would be in. This may be different in Turkey.
In the USA, what you would do is say something along the lines of "I am looking to apply for a job here. Could I speak with a manager?" Then you would find out whether they are hiring or not from the manager, who would also give you info on how to apply. No need to buy anyone lunch, unless it is a very difficult position to enter, and establishing a personal connection with the person that runs the place might help boost your chances. In general, for a retail position, just go in and ask who you would need to talk to about applying, and they will direct you accordingly.
I have done this to get my own retail job in the US, before I moved on from that industry, and it worked out well for me. This works well to get you to the manager, and once you have an interview that is where you can talk with the manager to boost your chances of getting the job by establishing yourself as a good fit.
Looking at your comments on the original question, it seems as though what you are really wanting to do is to get information about what the job is like, and how the job is actually performed, before you begin. You could always ask the employee that you mentioned something along the lines of: "What's it like working at a place like this? I'm interested in applying and was wondering if you might be able to share what your experience is like in the industry. Any pointers for someone that will be new to the field?" After the response you can gauge if the salesperson is really friendly or not on how to engage further.
For example, if they are very excitedly telling you many things about the position, maybe talk more about what got you interested in the field, and why you want to be in it. This can establish a good relationship and will make it easier to talk to the employee by making the engagement more casual. If this happens, then perhaps after you have applied and gotten the job, you can talk with them more during your training period, or set up a time to meet after work to talk about experiences. Setting up a time to meet outside of work, before you two are actually coworkers might be misconstrued, as you fear, as being interested in the person over the job.
When I was working retail, one of the overnight workers, that I only saw late at the end of my shift, was also working a job as a software developer (the job that I really wanted) during the day. At first, since I didn't know him well, it might have seemed odd for me to go up and ask him out of nowhere what his other job was like. It took a while, but after getting to know him a bit more, once the coworker relationship felt a bit more casual, I was able to ask him all about it, how to get started with it, and what to look out for. He even gave me a number to call after he got off work to talk some more about the different methods he had used to get the job he was in. Ultimately, he helped me get the confidence to move into the job that I have today, and I'll always have that connection for the future.
If the coworker seems very uninterested in talking to you at first, then you may need to, like I did in the situation above, wait a little while to build up a coworker relationship with them before they are willing to share their experiences. This is O.K. After all, if you really would like to work in the field, waiting a bit to learn more about it shouldn't be a problem. You'll be working it for a while, after all!
Ultimately, it would be easiest to learn about the job by actually getting the job, and asking about it while you are being trained. This is when you are supposed to be learning about it, and you will be able to ask questions (and get answers) about the job even from a coworker that wouldn't even dream of helping you out if you were a random guy that walked into the store off the street.
Introduce yourself, ask a broad question to start, and if you like what you hear, apply for the job and get all the information you'll ever need while you work it. You might even make a new friend that you can hang out with, and talk to about things unrelated to work! (you'll be able to get your "City 101" as you say in your comment, and perhaps even more.) After all, you'll be spending most all of your days with them, and that can make establishing a relationship quickly, easier.
Some of the best friends we make are the ones we make at work. Good luck!