First time, don't feel like an idiot.
It takes time and experience to be able to spot gender in different birds at a young age.
Call the guy and see what he says...chances are it's not going to be a problem to exchange or get another pullet.
I'd advise to bring home 2 or 3 tho because...
Consider the bio-hazard and integration issues with bringing in started birds....
..but medical quarantine time would be an excellent opportunity to spend time observing and hand feeding towards taming them down if needed.
Here's some notes I've taken on integration that I found to be very...