There are many running events in Paris throughout the year, but the Paris Marathon in April, the Paris Half-Marathon (Semi-Marathon de Paris) in March, and the 20km de Paris in October remain the most popular, selling out almost immediately.
Normally there’s no legitimate market for selling purchased bibs (because of injuries, illness or other commitments) or for those who missed out on buying them. There’s a black market, as described in this WSJ article Marathons, Triathlons Allow Runners to Resell or Transfer Entry Bibs, which besides opening up the possibility of scalpers buying bibs just to resell them at a profit, also presents the very real problem of what happens when someone has a medical emergency during a race wearing someone else’s name.
Certain races in the US are now offering refunds and transfers. In Paris, a new service, Luck-It! is an official website for buying or reselling your entry bibs. At the moment only the sold-out 20km de Paris is listed, and the website is only in French, but if you can figure it out then you’ll be able to get your money back if you can’t run, and get a bib in your own name if you’re looking to enter the race. Sign up as soon as possible if you want to get on the wait list (organized by gender and time expected to complete the race). You can also join the discussion on their FB page.