The Wall Street Journal recently published an article Marathons, Triathlons Allow Runners to Resell or Transfer Entry Bibs.
Bob Bickel, founder of RunSignup.com, says some 10% of events on the site scheduled for the next 12 months permit a runner to transfer a bib to another runner, up from less than 6% in the past 12 months. “People are kind of waking up to the fact that this is a good thing to do, rather than force your participants onto Craigslist,” Mr. Bickel says.
When the WSJ called, we did some research so we could talk knowledgeably. The raw data for any distance race for races in the past 12 months and the next 12 months is below.
The data is in 3 charts – 1. Past 12 months, 2, Next 12 months and 3. Races over 13 miles.
The two most interesting data points is that the % of races allowing bib exchange has grown rapidly from 5.7% the past year to 10% this coming year, and that the % goes up for longer (and more expensive) races.

Since the number of people who take advantage of this are typically far less than 1%, it might be worth the positive image for your race without impacting your finances negatively (and remember this can be a revenue source as well if you charge fees for transfers or deferrals).