Craigslist may have prohibitions... I've seen airsoft and paintball gun ads which state "THIS IS NOT A REAL GUN" or some such. If not explicitly prohibited, the CL content flag community may push the ad into oblivion by flagging as inappropriate.
Since it's your first post, I'll add the following:
Pay for the subscription and list it here in the classifieds, provide detailed pictures and commentary on condition, the location of your preferred Face To Face transfer ("FTF" above) etc.
If you are not a "gun guy" (i.e. this is inherited etc) then you technically cannot transport it in Massachusetts without a license. I don't know about other states restrictions.
In this case ask a friend with an LTC ("license to carry") to transport it for you.
If you are uneasy passing a firearm to a stranger, you can arrange for a transfer at a gun shop (aka "FFL" Federal Firearms Licensed dealer) and the dealer will execute the transfer and run a background check and Massachusetts FA-10 (firearms transfer form,) for a fee. The fee typically runs $20 - $50.
What state are you in?
Others will chime in to correct me if any of the above is wrong or misleading, or with other state-specific rules.
--Edit:
The subject says "in MA" so the above should be reasonably good information. As I said, if my limited understanding is likely to lead you astray others will correct me.
And "Go green" means subscribe here for access to post in the classifieds.