Catherine - around the Ft. Lauderdale area there is:
Arizona Shooting Range AKA Guns & Knives off of 441.
http://www.gunsandknives.com/
I know they sell plenty of new guns, not sure about used guns. I don't buy there though because I generally find their customer service to be not that great and prices to be just so-so.
There is Outdoor World of course in Dania, which is adjacent to Ft. Lauderdale and only a few miles from downtown Ft. Lauderdale. It is between Griffin Road and Stirling Road on Anglers Avenue just west of I-95. It can be seen from I-95.
My personal favorite, and where I buy most of my guns is IPS/911 Store in Hollywood, Florida - also very very close to Ft. Lauderdale, only maybe 10 miles from Downtown Ft. Lauderdale.
http://www.911storeandservices.com/welcome.shtml
They have new and used guns. They aren't very large though, and I wouldn't necessarily call them high traffic, although there are usually about 3 or 4 customers in the store at any given time. Very knowledgeable guys though. A large portion of their customer base from what I can tell are local PD officers and I know of one PD officer who I'm pretty sure is on staff there, although I've never asked him.
There is also Weston Arms which has a brand new location in Davie, Florida which is also very close to Ft. Lauderdale (all of the cities mentioned are in Broward County which Ft. Lauderdale is part of).
http://www.westonarms.us/
They have new and used guns. I don't know if their old location is still open. It would be worth a phone call to check. The new location which doesn't seem to be listed on their website is just off of I-595 on SR84 in between University Drive and Davie Road Extension on the south side of I-595. You can see it from the expressway.
If you give me more of an idea where in Ft. Lauderdale this person is, I could provide a better listing though. I know the area pretty well.
There are plenty of stores. I've just listed a few that I seem to prefer.
Also, as lacoochee mentioned, there is a gun show coming up at the War Memorial Auditorium in about 2 weeks. See here for location details:
http://suncoastgunshows.com/gun_shows/ft_lauderdale_gun_show.htm I am not a fan of buying a gun shows though. Mainly because I don't like buying guns that several thousand people have been dry firing for the last few hours. It's a high-traffic show though. If this is for somebody new to shooting entirely, it may not be a great idea as there is usually some nazi stuff at the shows and could be misunderstood.