Robnett, 25, was selected by the Athletics in the first round of the 2004 First-Year Player Draft. A left-handed hitter, he was limited to 82 games last season after early season surgery to remove a tumor from his stomach. He batted .236 at Triple-A, and has a .256 average in five Minor League seasons.
He was added to the Cubs' 40-man roster and will report to Spring Training camp.
Sellers, who turned 23 on Sunday, was selected by Oakland in the sixth round of the 2005 Draft. He spent an entire season at Double-A Midland for the first time in 2008, batting .255 with 15 doubles, eight triples, six home runs, 46 RBIs and 10 stolen bases in 123 games. He will report to the Cubs' Minor League camp this spring.
A middle infielder, Sellers played 63 games at shortstop and 60 games at second base in 2008.
The Cubs have been adding arms this offseason and decided to part with Wuertz, 30, who went 13-7 with one save and a 3.57 ERA in 265 relief appearances in all or part of five seasons with the Cubs.
He split the 2008 season between the Cubs and Triple-A Iowa, going 1-1 with a 3.63 ERA in 45 relief appearances for the Cubs.
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.