Bus Ticket / Email Compiler

We have a bus ticket company that sells point to point bus tickets. Someone goes online, purchases a ticket from the site, and one of the many bus companies runs the busses. Each bus has about 50 seats and the problem we run into is that when different people use different websites to purchase the tickets, we can't manage how many seats are left on each bus . For example, Website A sells 30 tickets, Website B sells 10 tickets, Website C sells 13 tickets. By now we are overbooked. We need a system that can track tickets from multiple vendors (mabye by parsing their emails like done with TripIt).