Does advice like this apply in some way to personal/career development?

I would like to break in to a particular technical field. I believe that to work in that field, I'll need to demonstrate some measure of experience in that field. To that end, I'll need to build. This feels like a 'build first, sell later' kind of approach.

Or should I tip the scale toward 'selling' myself on the basis of my other useful experience, and build my expertise on the job?