A fluted barrel is considered to be more accurate as they are usually longer, thicker to allow for the flutes, and thus stiffer, which increases accuracy, and because of the flutes, it will cut down on the weight of the thicker barrel. Also another advantage firing multiple shots, you have more surface area to allow for cooling between shots.
As for what's the advantage in buying used off the classifieds, I would say they are many, that is if you don't mind tinkering, and know how to fit a barrel by following the FAQ's section on fitting barrels.
The main advantage is you don't have to wait weeks and sometimes months to have you gun returned, you also have the satifaction of fitting the barrel just the way you want it to fit, and you get lucky sometimes and don't have to do a thing at all but place the barrel on the frame, attacth the forend, and go buy some shells or reload them and head for the range, within just a few days of buying the barrel.
Buying a used barrel isn't for everyone, but most here that do, will tell you it isn't that difficult to fit one, if you just work slow and follow the instructions.
Good luck
