For sale, or on sale (that is, discounted); more generally, available. First cited in the OED in a (London) Daily News in 1881: “Old wheat scarce and dear. Very little barley on offer.”
The Google Ngram chart below shows the use of the expression in American English from 1900 through 2008, with the relentless increase commencing in about 1972.
Tens of thousands of Apple Macintosh users visited the Macworld trade exposition here earlier this month, examining the hardware and software on offer.(Peter H. Lewis, New York Times, April 22, 1990)/Self-improvement has always found a ready market, and most of what’s on offer is simply one-on-one instruction to get amateurs through the essentials. (Atul Gawande, The New Yorker, October 3, 2011)