They buy a lot of pre-packaged food that can be sold for cash.
Canned foods are the favorite, especially meats. A bud of mine in the FBI once broke up a ring which was hijacking and selling mostly canned tuna-fish. Long shelf life, durable packaging, relatively small size, national brand names and not so easy to trace.
The amateurs, of course, go after anything to turn a quick buck. The pros are a lot more selective.
I sometimes wonder if some of that really bargained priced canned goods which you can buy at places like Big Lots have been laundered through such a system. We're talking big business here. My bud in the FBI says they think is may be conservatively about 20% of the size of the above ground economy. More generous estimates range as high as 50%. That's one hell of a lot of taxes which are being evaded . . . and a number which can only grow as the government's take increases.