Price depends on whether or not you mind selling them knowingly for food. I start my roos at $10 and if someone is looking for a flock roo or breeder, I will sell him for $5.
Hot2Pot gave great advice, I will add to it a little. I also pick the bird up and check for a sharp keel bone (the center bone in the chest) because the keel can be felt and should have good mass on either side at a minimum, a sharp keel indicates the bird is too thin and could be caused by worms or illness. Then I look at the bird's vent area to determine how dirty their bottom is and get an idea of how their poo is, I also look for parasites by separating the feathers near the vent, base of tail, back and neck area so I can see if there are any creepy crawlies on the bird. Carry a bottle of purex hand sanitizer with you and clean your hands between vendors. And do not forget to quarantine anything that comes back home, new or one you did not sell.
Good luck and have fun.