Likely, rather than putting out a public post, the person is contacting folks that they think they want to boat the GC with, and who they think are likely to be going this year. It's highly doubtful they have a way of knowing for certain who has a permit.
That said, I don't really consider it spam, but see it as an invitation to work a deal with someone for a win-win situation for two trips in two years. If you don't like it, don't reply. If you have a trip this year and also want to go in 2014 but don't have a trip yet, then you can get back to them and start the interview process to see if it would be a good fit.
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse