Agree with above poster. The last couple weeks of the season are the best time to buy non-trainee players. The "ineligible for playoffs" tag scares people away, and many don't like to add salary at a time when they are not making as much money. However, right after the draft is also a good time to buy non-trainees because many people are spending their money on trainees at that time and ignoring the more experienced players.
Basically, any time in the next week or so is a decent time to be buying some experienced players...just know what stats you want for what price and wait for the right player to come along.
http://www.youtube.com/watch?v=oHg5SJYRHA0