utahgunexchange has some great stuff from time to time but you have to wade through a bunch of garbage.
gunbroker is easier than you think. You create an account, buy it like ebay, then send the seller the info of your gun guy. They send it to him, boom, done. I buy and sell quite a bit that way.

What are you looking to do with the 243? I've always liked that cartridge. With light bullets it's like a 22-250. Heavier bullets and you can take deer.