The more professional, the better. Most people in the industry know what stock is used for and how to make it more available for using it out of context. If you take city scapes, I would presume you take some of menacing looking buildings with crazy angles. You probably use photoshop to edit out power lines and re-touch things. You are prepared to lose the ability to say how your photos are exactly used and receive payment for them.
It's not really for the folks who go and randomly take a few pictures once in awhile. It's for those who go and photograph things for clients, for art, for resale as generic stock, and for resale as newsworthy images of current events. Your work would likely be on a website somewhere to showcase your talent. Do you have talent?
I have some talent, but I am definitely a casual photographer. So rather than sign up, I will just include a line in my image credit when I'm ok with them being reused. Is there a tag one could use when posting photos s/he is willing to have used as stock?
You could tag the entire post as #stockphotos (like this post) which will put it in the correct category. Otherwise, make an effort to learn about Creative Commons and how licensing works. Just a made-up tag for permission is not recommended. Many people are using this "CC" followed by numbers and abbreviations for what things are permitted.
Once you choose a CC license, make it clear that your photos are licensed as such.
When the site is up and running, I will make sure this same choice is available and try to steer people towards licensing that permit you to make money from the use of your work.
All major image "sharing" and storing websites permit some form of licensing, although some places have been known to default to permissiveness that is only attractive to the website owners, not the artists.