For my first road bike i bought a used Bianchi for about 1/3 the new price. I bought it off ebay and the seller wasn't completely honest about the condition.
I was able to fix the front hub (which was badly worn) for about $50 and then the bike was roadworthy. It was a great deal considering I wasn't sure I wanted a road bike.
YOu already described the #1 reason not to get a used bike. It's hard to find the right size. but if you do? I'd buy used again in a heartbeat!

ps after riding the Bianchi for about 2 years I sold it again for $50 less than the price I paid for it.