Could be money laundering or drug sales? Pay tens of pounds for a load of rubbish and when it arrives, the package also contains your drugs - I've just made that up BTW, I don't know if drug dealers actually do that.
Or it could be 'work' in support of their universal credit claim.
For people in the right circumstances, eg 3+ children, this could be worth hundreds of pounds a month in top up benefits, possibly more. I think they were in the process of phasing it out by introducing the minimum income floor, where, they would assume that your business earned you NMW for the hours you were required to work, even if it didn't, so reduced the top ups received people who pottered round car boots and put bits and pieces on ebay or did their mates nails in their living rooms for a fiver a time and claimed to be working as a beautician.