You do not need to be in a big gang to make money from Crime. There are smaller thefts available which can provide a nice burst of money with very little effort required, just be in the right place at the right time.
All 4 main corporations (Not APD) can be stolen from via their corporation till. You will need a bat with at least 50% durability to break open the till and begin taking any money that has not been moved by the corporations Senior team.
The longer you stay the more chance there is to be caught by the cops so judge it right. Bear in mind, it may be easier to steal from Corporations with less active users, however the sales into those tills will also be lower!
There are 2 stores, the Habbo Store and the Arcade Store which have a safe that can be stolen from.
You will need lockpicks to attempt to break open the safe as well as enough time before the store owner calls the police.
You can earn any amount from 100c - 165c. There is then a chance of doubling to 200c - 330c. There is then a chance of tripling to 600c - 990c
If you have managed to get your hands on stolen heist items, prison keys or APD gear head over to the public blackmarket where you can sell these things at a discounted rate but without the need of a turf.
The public blackmarket is located in a sewer below the Criminal Court and the gang there will purchase your stolen items for a decent price!