Inside an existing building?
Building the building itself?
How many shelf units?
Probable material will be white oak
Ease of access in and out of location?
We have more questions than you do, and I have not even started.
My suggestion is to walk away. You are too unprepared.
I think this is harsh. You gotta start somewhere. I suggest giving the prospective buyer a verbal estimate range. Tell the buyer that it depends a lot on the details and you can provide an accurate estimate in writing once you know what they are.
-> Maybe try a verbal $250-400 range estimate and see what happens? Remind the prospective customer those are common prices that sell on the internet, and your quality is much better than the junk you see on the internet (hopefully true!). Remind them that you get what you pay for, and quality and workmanship cost money.
You may make or lose money on the deal, but that's how you acquire the knowledge to make better estimates in the future. Good judgement comes from experience. Experience comes from bad judgement.
People tend to latch on to what you tell them first. It is better to guess high, and you still may not get it right. When I worked for companies, I hated when bosses would demand a quick hip-shot guess without giving me sufficient information about the job. After that, you learn more details, discover the pitfalls, and your boss increases the scope of work. Sadly the boss almost always puts the original guess on the actual schedule, of course.
... and that doesn't begin to consider pushback you get when the boss doesn't like your guess or more accurate estimate.
I have never sold any of my woodworking projects. I have made many estimates for other kinds of work, and hate estimation more than any other aspect of those jobs.