Saying you are "tax exempt" is a little vague. It can mean 3 different things:
1.
Exempt from paying federal income tax.
This is automatic for 501(c)(3) groups, but it isn't really relevant here so I won't go into it.
2.
Exempt from paying sales tax on your group's purchases.
This one varies by state, and is not automatic with a #1 exemption. It
may be automatic in some states, but as far as I know there's usually a separate application process. This exemption is probably not relevant for the book fair so 'nuff said.
3.
Exempt from collecting and remitting sales tax on items your group sells.
This is the one you're concerned with for the book fair sales. The rules vary by state. Having a #1 or #2 exemption may or may not automatically give you a #3 exemption. It depends on the state. I'm not familiar at all with NY tax laws so I'm not much help, but taking a quick look over this
Guide To Sales Tax In New York State
I don't see anything to indicate that non-profits are exempt from collecting tax, so I'm
guessing that you are supposed to charge tax. But I stress
guessing. Please double check. You could probably get a definitive answer by calling the Dept. of Tax & Finance or doing some searching on their web site:
Department of Taxation and Finance
Like someone else mentioned, even if you are required to charge tax, you do have the option of not collecting tax from the puchasers and eating the cost yourself (ie paying it out of your group's funds). It's the remitting part that the state cares about, not so much the collecting part. Generally, states don't care where/who the sales tax money comes from as long they get it