This is a complicated topic because there are to many variations of why people buy old cars. If someone is looking for a common old car like a standard Cutlass, Camaro, or Mustang, they are all over the place all across the country. Typically can find a nice one local and the local market may be different than somewhere else in the country. If you are looking for a rare car or a very high end specialty then you had better be looking nation wide and that market is pretty consistent across the country because of what it is you are looking for.
On another note, People like me, I have to consider every aspect of a car before buying it and that includes getting it home. I simply can not afford this hobby and only have so much money to play with. I have to consider the price, getting it home, what I need to put into it and what the final value will be. I enjoy restoring cars as much or more than driving the finished product so I have to get it at a price that I will not lose money when I am moving to the next project. If a car is worth it and I am safe to not loose money I don't try to talk someone down. If it is to high for me and been on the market for a good while and not moving, I will toss in all the cards to get it for the right price. Yes, I have even tossed in the "I have to get it home" factor a few times but I am also not buying from a dealer or professional. When the car has been on the market for a while it is because nobody wants it or it is over priced or both. That is when I move in and use anything that works and if it don't work I walk.
There are so many aspects to this but all of it is fun. It would be more fun if I could keep all my cars and not have to sell them.