Things to consider when purchasing a used tank,
1) Check to make sure all of the silicone is solid! Even if its not you CAN re-silicone an aquarium easily. Also, look to see if the silicone is stained. If so it could point to the previous owner using a chemical treatment in the tank (such as what might be used to get rid of ich) that could leach into the water of your new SW tank, something you wouldnt want to happen. Hopefully someone more knowledgeable here can tell you more about the leaching because I know very little about it personally but was warned about it.
2) Scratches. Its pretty hard to tell looking at a tank without water in it. The 29G reef tank I have was bought used and has several annoying scratches. Take a good look at the tank your considering buying to ensure its not covered with scratches. If its an acrylic tank they can be buffed out but with glass your stuck with them.
Regarding a first time saltwater tank BIGGER is always better. The reason for this is water volume. A gallon of evaporated water from a 29G would raise the salinity of the aquarium FAR more than it would with a 75G aquarium (2.5 times more), and would be deadly to all inhabitants of the tank if no one was around to top off. Same goes for toxins. Its far easier to maintain stability in a larger marine tank than it is a small one. I started with a 29G and still have issues keeping my salinity stable, in fact I spent all day making small adjustments to the salinity in the form of small water changes to slowly raise my salinity. I still need to work on it tomorrow.
I know it sounds big you should really start with a 75G or larger. After reading a multitude of threads on here and other forums I though I could easily pull of a 29G reef against my LFS recommendation of a 75G starter aquarium. Im sure my tank will be a success but I do have to top off the tank daily and make adjustments way more than I would have to with a 75. In fact, Im ordering a new 75G in about a week. Ive already got $1200 in my 29G build which could have been used for the new tank had I listened to what I was told.
Listen to the advice of what these folks here on this forum have to tell you and you will have a great time with your marine tank. Good luck, Im looking forward to watching the build.