I don't know about other brands, but IIRC, my Specialized Allez had a weight limit of 250 lbs. If you buy new, make sure to check this out on any bike you're interested in.
Concerning the comment on Craigslist, there are good sellers on Craigslist and bad ones. There are junk bikes listed there and there are very high quality bikes listed there and lots of bikes in between. Saying that all bikes on Craigslist are junk is completely untrue. That said, there are some great bargains on Craigslist that can save you to ton of money, you just have to know what you're looking for.