The auction, scheduled to run for 10 days, reopened Monday with an opening bid of $7,500, D'Army Bailey, the Memphis judge who owns the tub, said on Friday.
A portion of the sale's proceeds are to benefit the Boys and Girls Clubs of Memphis, he said.
The tub first went on sale in July 2004 and drew a bid of $152,000, but eBay removed it over concerns the item might be considered offensive, particularly to King's family.
But the Web site's operators later decided the item's historic value met its guidelines. It was put up for auction a second time, but did not sell.
Bailey, one of the founders of the National Civil Rights Museum located at the site of King's death, said he got the tub in 1983.
King was shot on the balcony of The Lorraine Motel while in Memphis to help lead a strike by city sanitation workers in 1968.