Yep. One of my friends who I thought was more intelligent bought one of those from ebay. She ended up paying more than what it would have cost to book two cruises! She paid something like $1600, and the two cruises would have cost her only $1300.
You're very lucky you were beat out by 8 cents!
Go by a travel agency (or call Carnival 800 number) and ask for one of their new brochures. There's always the 'list price' and the 'super saver price'. NOBODY pays the list price.
Find a cruise you'd like to go on. Check a few sources. Check with a local travel agency, an online agency, and also call Carnival and speak with a PVP (Personal Vacation Planner). You're under no obligation to book with any of them, just check the rates. Make sure you ask for the bottom line price, as some agencies give you a quote, then say "oh yes, then we add the port charges and taxes." Carnival PVP will give you the rock bottom price. When you speak with a PVP, ask about any specials that are going on at the time.....like state of residency discount, or past guest discount. They are out there...and sometimes you just have to ask.
Once you're armed with all the information from the different booking agencies, including Carnival, then you'll be ready to make an intelligent decision as to with whom to book. Good luck.