$50 dollars for a reconditioned ball lock keg is not a steal, but depending on where you live is not a crazy price.
Even if you only get the 30% deal and my basic math skills don't fail me. Thats $35 a keg and that is not bad at all in my book. Unless they charge unusually high shipping. I don't think that company would do that though, you'll have to check. Either way you'' pay shipping.
If you can get the 50% off deal $25 per five gallon ball lock keg is a great price (as long as the shipping isn't outrageous)
There are a few differences between Pin-locks and Ball-Locks. Pin-lock refers to the type of post the in/out lines are connected to and do not have a manual pressure release valve, just a set pressure relief valve.. ie, certain pressure it lets go.
Ball locks have a different type post for the in/out fittings but do have a manually operated pressure relief valve. It comes in handy when you want to purge the keg of oxygen. A pin-lock can also be vented manually. Just by a different method.
Pin-locks cost less than ball locks. I prefer ball locks. Somepeople are very happy with pin-locks.
In the end, you want all the same type of kegs in your kegerator so you don't have to change out the quick disconnects (the part that attaches to the post) each time you change between a ball-lock or a pin lock keg.
There is also a slight difference in their dimemsions. Pinlocks are a bit shorter, maybe 1/2 inch and a little bit wider than ball locks. Sometimes the size is the determining factor in a kegorator setup. I hate to say search to take a look at both of them. Going to post this and then look up pics and dimensions of both and post back here unless somebody else already has.