In a post, write a line of poetry that seems to be an opening line. Discuss why your line is a first line rather than a line that might function equally well in a different context. Describe the poem you envision following your line in terms of ideas and form (stanzaic structure, meter, rhyme, length, tone, etc.). Then be prepared to give your line away.
Now in a comment below the post, other students “offer” suggestions – what sort of poem do they see burgeoning from that opening line, and why. These comments form the “bids.” Then the author of the line agrees to give it – no strings attached and forever – to the student whose suggestion, enthusiasm, or oddball approach most pleases him/her.
Each student produces a poem with a “purchased” opening line.