I actually booked two cabins on the Explorer last week after placing an ad. The agency that responded with the best price was Karrol and Dave Angell of Cruises & Tours Unlimited. Tel: 888-649-5176. I spoke with Dave and mentioned my concern about dealing with an agency that I didn't know was legit or not. So he recommended I call BBB and Royal Caribbean, which I did.
This agency was registered in good standing with BBB and had no complaints to date. But what probably alleviated my fears most was just speaking with Dave. He and his wife are retired in Jacksonville, FL and work for Cruises and Tours Unlimited. He had visited the Explorer and was very helpful with cabin selection. We had a nice chat and everytime I called I got either Dave or his wife not a recording. Also he got back to me immediately with a confirmation and cabin #'s, something that doesn't seem to happen much anymore. Two days later I received the paper confirmation with the same price that was quoted. So far, very satisfied.