While fit is the most important factor, it certainly isn't the only factor.
I'm far from a pro when it comes to this stuff, but I have made some observations over the past few years. Please feel free to take the advice below with a grain of salt, and note that none of these "rules" are hard & fast. They certainly wouldn't apply to touring or MTB frames, so please read this entire post in the context of a steel road racing frame only.
If you're looking for steel road frame on
ebay, there are a few things to look for:
- Forged dropouts - Avoid stamped ones. You can easily spot these,
- Bent tubes/fork - Look for bends/kinks near the head tube,
- Geometry - tighter clearances between the tyre & frame can indicate better quality. If it looks like you could fit your fist between the seat tube and the rear tyre, then it's probably a cheap/low quality frame. A thumb's width would be more acceptable (a good "rule of thumb!"

).
If the seller doesn't provide enough photos to enable you to take in these details, then ignore it and move onto the next one. Also, ignore anything from interstate/overseas. There are millions of bikes around. I guarantee that you will find something you like within 15km of your house.
For some pictures of desirable frames, or frames which have the attributes you should watch for, you can look at websites like Fyxomatosis.
Most importantly: Don't rush it, and have fun learning.
