Allow bib transfers?
Many runners view themselves more as ticket-holders than as event participants and want to be able to sell or transfer an entry if they cannot run a race. This is especially true in long-distance events that close early, forcing runners to pay an entry fee long before they know if they'll actually be able to run the race.
The LaSalle Bank Chicago Marathon closed out at 45,000 runners six months before race day. Grandma's Marathon, in Duluth MN, and the Marine Corps Marathon, in Washington D.C. also close their fields long before race day.
The Gasparilla Distance Classic allows runners to switch to the 5k or defer their entry to the following year if they can't run the 15k distance. But they don't offer refunds.
You are not obligated to transfer an entry or refund a fee to a runner who is injured or cannot participate for another reason. Be aware, though, that an increasing number of runners expect race directors to be able to utilize technology to make these transfers. If you want to meet customer expectations and stand out from the other events, it may be a good idea to create a way to allow bib transfers.