Depending on draft settings, your league may or may not allow clones in your league. If clones are not allowed, then previously drafted players will not appear in the player search. For instance, if another team in the league has already selected 1981 Mike Schmidt and the league does not allow player clones, then your search for a 3B will not return any instances of Mike Schmidt.
Theme League Drafts also enforce the salary cap when you make your player selections. So, if you only have $500,000 available you can't draft a $600,000 player.
Taking it a step further, if you only have $500,000 left with two open roster spots remaining, you won't be able to draft a $400,000 player because the cheapest player in the pool is $200,000. For draft leagues where career players are available, the minimum remaining balance per player left to be drafted must be $400,000 because there are no $200,000 career players and only a few others around that value.