There are  a variety of paint guns offered on Ebay.  Everything from professional ones selling for hundreds of dollars to inexpensive ones for twenty or thirty dollars.  If indeed you make your daily living doing auto body work than it makes sense to buy the best available, but if you only occasionally paint it's tough to justify a huge investment.    Paint guns have evolved from those used in the 1970's and 80's, as has the paint used on cars although the old type gun is still available.  Part of the change is due to enviormental concerns which is how the high volume low pressure gun (HVLP) came into being.  Some old painters still hate them and refuse to use them.  By lowering the air pressure and a cone of air holes they guide the paint from the nozzle with less overspray and mist in the air.  They don't get all the paint to the surface, but they do use less paint, and given the high cost of automotive paint that's a plus, they also use less air so a smaller compressor will work when using them, but I'd still want a large tank.  My paint supplier now says there's a new HVMP gun made, as in medium pressure, haven't tried those, but they are a sign that HVLP guns can be tricky to use. 
So will the inexpensive guns sold on ebay work?   Here's what to look for, first of all you want an air regulator on the gun, one that has a diaphram and gives constant pressure, not just on on/off valve.  I use and prefer gravity feed guns, those that have the cup above the gun rather than the tank below the gun.  One critical part of the gun is at it's nozzle.  The rod that fits into it must be fit correctly  or the gun will drip paint ruining any spray job.  Look at the front of the nozzle and make sure the tip is centered not off to the side and that it works correctly when the trigger is pulled and released, if the rod is bent or doesn't fit there's little that can be done to correct it.  Since this isn't possible to check until you see the gun make sure the seller will allow a return or will send a new nozzle and rod to correct the problem.  The next thing to do is check over the gun and make sure the nuts and screws are tight but not too tight on the packing nuts.  I oil the air valve and trigger and rod, I lightly grease the spring inside the trigger adjustment. 
I use three guns when painting, one for primer, one for base coat and one for clear.  You can use the same gun for base and clear but the pressure will have to be changed between the two.  Primer is hard on a gun so I have one with a 1.7 nozzle dedicated to just that for the other work I use a 1.4 nozzle.  Base coat / clear coat is much easier to use if you don't have the luxury of a paint booth.  The base coat can be shot at a lower pressure than the clear.  The clear coat can be tricky since it will need more pressure to go on smooth.  I thin the clear coat with reducer to make it flow even better and adjust the pressure up to get the best results.  Every paint gun, no matter what price needs to be cleaned properly, after using run thinner through the gun at least twice and clean front air horns and nozzle removing it to soak.
Aside from guns, please remember that  paint is extremely toxic and dangerous.  Don't shoot it without at least a new double cartridge mask or fresh air system.  Cover your body to protect it from the spray.   Limit your exposure to it as much as possible .   Modern day  paints  can cause horrible health problems and  death.