Actually A.B. says your grandma's cast-iron skillet will actually work better than a new one. I think because it's already been seasoned.
True! And if you buy one secondhand that was mistreated, it will stick and burn things horribly, very much like a badly treated Wok. You can season it and start over, but it takes awhile for them to be awesome again.
Cast Iron Skillets should never wear out so finding a good one at a thrift store would be awesome indeed. It should feel VERY heavy for it's size. If you find one with a layer of rust but in otherwise good condition, take the steel wool to it and get the rust off. I season my new ones by rubbing them all over (inside and out) with crisco and putting them in the barbecue grill with a hot fire going for like an hour. It should smoke a lot. Once cool, they go inside and on the stove, get heated up again and the inside gets rubbed with oil. Then cooled. Extra oil wiped out and ready to go.
Don't stack any other pans on or in your cast-iron stuff, it will make them rust. I keep a paper towel in the cast-iron pots and pans to absorb any moisture that might get in there. Once they are nice and black and shiny you will be very happy with them.