"many retailers have been using the try-then-buy tactic for years"

Many sites also try to do this - allow the user to perform an action (rating, create a playlist, post a comment, ...) without having to commit to creating an account.

It would have been interesting if there was a study with how time between handling and bidding, as it might help with the problem of maintaining engagement of new users.

Keeping in front of the user's eyes using email or inserting themselves into their current interactions can help remind the users to use the new service, but there is a fine line (different for every user) between informing and annoying.