I've build up number of bikes. Things to remember:

If you buy a frame off e-bay, does it come with a fork or not and what about the headset? Brand new from manufacturer, you will get a fork and frame but generally no head set unless its integrated.

steer tube comes in 1" or 1-1/8" mostly non-threaded these days.

Steer tubes will require some cutting.

seat post also comes in several sizes so do pay atention. This will determine the size of the clamp on type front deraillure. And if the front deraillure is a hanger type there are two sizes as well. And if it is a hanger type you might need to check if it will be compatible to a compact double chain ring.

Bottom bracket. you again have two choices. This will be determined by the frame English or Itallian. so if you do buy a bottom bracket make sure it is comaptible to the frame.

Oh the stem and handle bars. Again there is the regular size and the oversized.

But most important of them all. If you do build up your own bike, you get a bigger bang for your buck with good rear deraillure, and good set of wheels, followed by bottom bracket.

And generally it is much cheaper to buy the complete groupo or mixed groupo then buying one piece at a time.

mixed groupo would be something like DA rear deraillure, DA shifter/brake set, ultegra front deraillure, cassette, hyperglyde chain...

If you need help or questions, please PM me. Also the other person who can help you would be DebW. She knows far more than me. If you are not sure about the cost of an item, like is it a good dal or not again PM me,

It really isn't all that hard to do. just be patient and be cautious. And have lots of fun.
smilingcat