You could just grab a book and toss it out into cyberspace, but it will be helpful in the long run to pause and think about the process.
Do you have a suitable name? Will the library connection be clear to the potential customer? Many people will happily support a library when making a choice among multiple similar copies. You want to claim that good will with your seller name.
In order to register and create an account with any online site you will need a credit card. If your group doesn’t have one, ask the bank which holds your account to issue one for the organization’s use. To sell on Ebay you will need to create a PayPal account.
Consider record keeping. What are the sales tax requirements in your state? You may not have to collect tax for a once a year sales event but likely will when your sales are all year long.
A computer is enough to get started and will be all you need for small collections. If you are working with a large volume of material, especially recent bar-coded books, a scanner is very useful.
You’ll need shelving and a system for filing the books offered for sale.
Identify sources of packaging materials and a space for wrapping packages. Let people know that you want used boxes and peanuts/bubble wrap and you’ll probably get much of what you need for free.