Our school did a physical book fair with Scholastic with an online companion that lasted about 2-3 weeks afterwards.

You could also do an amazon book list. I know our school has an agreement that if you go through a link on the page you can get a kickback.

Scholastic just started doing an online book fair this past year. Not sure if they will allow it to be run without a physical book fair or if they make you have an actual fair.

Bookworm, a company based in Manassas, does online fairs. We've only used them for our in-school fair, but we've been very happy with their responsiveness and selection.

Bookworms runs fairs for many local schools including (I'm pretty sure) Beauvoir, St Pat's, Norwood, Landon.

Our school is thinking of doing an online book fair. Does anyone have experience with this and could recommend a vendor? Thanks!