To answer your question on where to search, consider the following.
Big active countries (ie china, netherlands) have lots more players on the market & generally better players, but a lot more competition, so players tend to be sold at higher prices.
Smaller countries (ie Australia) have a lot less activity/players for sale, so they tend to be cheaper & your win chances are higher.
But as the game progresses & people are able to search more markets, the competition in the smaller markets is increasing.
Personally, the last 2 trade periods I have mainly searched the bigger markets, due to the quality/numbers of players available. But during season 1 I found some of the smaller markets better hunting.