Questions in regards to a Facebook account?

Firstly, i don't do Facebook, and lately more and more people, friends and others, that i have done business with, have asked me if i sell my brake, and suspension parts on Facebook marketplace.
I clicked on their page, thinking i might set myself up with an account, on that site.
Got to the point on the page that they are asking me for my birth date.
Why do they want to know my birthday?
What use is that information to them?
For you guys that do Facebook, did you give them your real birth day, or did you give them a fictitious one?
Just asking here, to which way that i want to go, with that site.
Thank's a lot.
Jim V.
hemi71x