Are there any regulations baring shorting these 'shadily marketed stocks' ?
In the markets I'm most familiar with, Canadian, the answer is no.
To be honest the biggest hurdle you'll come across is how do you short penny stocks that are being pumped and dumped?
1) There will often be no options on these so you can't use that avenue
2) Where will you get borrow on the stock, if its a penny stock, getting the borrow to short can be tough.
3) If you can get borrow, its likely to be expensive, Can you afford to hang onto the borrow long enough to cover your short?
If after all that you can short the stock then you need to consider, do you have the stomach to short and watch the stock rise due to the "pump"/hype the stock is getting?
Will you close the short after your first margin call? What about your second one?