I'd be skeptical of this, I can't think of many reasons why someone would sell GCs at 60% of value on Craigslist.

Besides the previously mentioned travel credit reason, there aren't many reasons why someone would do so. The only other (semi) legitimate reason I can think of is an employee has a travel budget they are assigned for the year, and are buying GCs to submit an airline charge to their employer. If the employer doesn't actually check that said charge is a flight, the seller could theoretically be cashing out their travel fund. The person would still be screwing their employer.

Either way though this sounds beyond sketchy.