As a writer with an agent, should I post my book in the "available rights" area of Publisher's Marketplace.com? Does it give the book any more visibility, i.e. do editors look there?
You have an agent. Why are you trying to sell your book?
If your agent puts it up on available rights, editors can and will see it.
Ask her to.
It doesn't cost anything.
I've had several nice juicy bites on the rights offerings list.
If any of my clients started posting rights offerings independently I'd have a conversation with them that would involve their ass in a sling.
I keep track of every place I send a project, who's said no, why, leads, all that stuff. You start sending stuff out to editors on your own and it's going to get very confusing very quickly. Plus it makes me look like an idiot and frankly, I can do that just fine on my own.
Leave the selling to your agent. That's why you pay him/her/it the big bucks.