Is craigslist part of the Shadow Economy? Is craigslist part of the Shadow Economy? Many if not most of the purchases that take place there are done without taxation.
If we take the definition of shadow economy to be illicit economic activity existing alongside the traditional economy and further pin down illicit to mean any sort of economic activity that violates laws governing our conduct (civil, economic etc.) then the short answer is - yes. 

Why? Many reasons. I think the easiest to verify and perhaps most obvious is that the majority of gigs, services, temporary labor positions, etc. on Craigslist are paid in cash. Here I make an assumption: most of the people doing these jobs for cash are not reporting these extra earnings. Thus, they are generating a supplemental income that goes untaxed. 

I read a bit of (non-academic) research about how these sorts of sites often provide a lifeline to illegal immigrants precisely because they facilitate economic illicit activity (I will update this post with the link if I can dig it up).

Why else? Again I speculate (but reasonably). Craigslist, I think, is often used to move stolen property. Because transactions are largely anonymous and conducted on a cash basis and with little or no record of sale, CL is a safe place for petty criminals to liquidate stolen assets. This also clearly aligns with our given definition. 

You could probably think of other reasons to support the claim that CL fuels (alternatively: is part of) the shadow economy in America. Then again, I think these two examples make it clear (if you are willing to accept the assumptions throughout the post).