Most of the time, in most leagues, not only is it "better" from a competitiveness standpoint to make a trade, I've found it generally better for your team.

Here's a suggestion: Go to an online baseball page like ESPN or CBS Sportsline and access the players at the position you are short. Next, as you look through the list from the web page, use the search feature in the League homepage to find players that appeal to you. Don't necessarily try to make a run at superstars--in PB, as in real baseball, there are usually useful journeymen sitting on somebody's bench.
When you see a player who interests you, look at that team's roster and compare it to your own for possible trade matches. This process will often yield you a player you might actually want to keep, rather than an INN space filler you will dump at the end of the year.
It also encourages trading activity in your league and often helps you become familiar with the trading practices and personalities of the other managers in your league.
Frankly, I've found that when you are rebuilding, some managers will cut you a better deal to help get your team back on its feet.

Dave N