I have unraveled several sweaters from a thrift store. Some worsted weight but most sport/DK/Fingering. It is great fun
just make sure to look at the seams to ensure they are sewn together. Some sweaters are cut from a large piece of cloth and you end up with tons of single strands! Have fun, I have taken apart several angora (rabbit) sweaters and they shed a bit but it didn't ruin the yarn I ended up with. Just be careful when pulling it apart so you don't break it. If it is more than 50% animal fiber and is not a "superwash" then you will be able to felt the ends together for a seamless join
HAVE FUN AND HAPPY KNITTING!!
yes I yelled! Thought it was appropriate