unlike football and basketball, players don't declare themselves to be eligible for the draft, they are eligible in certain years. i'm sure someone can add more to this, but all high school seniors are eligible to be drafted in the MLB draft. if you are not drafted or do not sign directly out of high school, you will again be eligible three years later and i believe every subsequent draft while you still have college eligibility. tons of players are drafted twice, and theoretically, i think a guy could be drafted up to 4 times. HS senior, then after his 3rd, 4th and 5th year of college. i've got a buddy who was drafted in the 2001 draft as a HS senior in the 19th round by the marlins, he decided on college. he pitched three years in college and was drafted in the 6th round by the white sox in the 2004 draft. three years of college improved his stock by 13 rounds. lord knows how many mid-round high schoolers sign only to be out of baseball in a year or two with no more amateur eligibility.