What are some red flags to look for when buying a used road bike? I will be buying my first road bike (after 15 years with the same mountain bike) and I have found many for sale on Kijiji. When I go look at the bikes, what should be some red flags that turn me away from them?

I have already researched brands, but are there warning signs when looking at the bikes?
Make sure that the frame is not compromised. Looks for cracks or big dents, or any asymmetry in the forks.
Make sure that there is no rust through the paint anywhere. If there is any chrome, make sure that any rust is only on the surface.
Make sure that the seatpost isn't seized. This is pretty easy as you can just loosen the seatpost bolt and give the saddle a squiggle. If it moves, you're golden.
If the bike has a quill stem, make sure it isn't seized up. This requires loosening the stem bolt, and whacking it with a hammer or preferably a soft-ended mallet.
Check that the bottom bracket and hubs turn smoothly. If the motion is stiff or jerky it probably means the bearings or races are shot, and that they'll need work.
Ensure that the cables work. Try the brakes and try the gears.
Check the braking surfaces. Most wheels will have an indicator groove along the center of the braking surface. If this looks quite worn it means that the wheel is on its last legs.
Don't worry about dust. Dust is a good sign, in my opinion. It means that the bike was probably stored indoors and by an owner who didn't use it much. These are good things.
Don't worry too much about brakes pads or tires. You can replace those fairly cheaply.
Some old road bikes have really gummy brake hoods that can be a pain to deal with. This isn't really a deal breaker though.