I understand that there are restrictions on advertising a HUD home you have under contract, but what are they specifically?

Just got an accepted offer of 70k yesterday on a 2 bedroom condo w/o rental restrictions.

Second lien buyers may have additional restrictions and calculations when it comes to ITV."

In most cases, there will be a deed restriction preventing the buyer from selling or using the property as an investment.  

You may also face other restrictions as @Alex Pereira pointed out.

Overnight the HOA can turn and suddenly you are restricted from everything other than owner occupied status.Regardless of the projected cash flow, I would never buy into a condo with an HOA.

It is not uncommon for a lender to pose restrictions on subordinate financing.  

They might have restrictions about self representation that make you use another Broker in your office on your transactions. 

You are limited by the 0 down restriction...direct mail marketing, which is a mainstay of wholesale real estate, costs money, even if you hand write and hand gather addresses. 

If you expect to deal with those that hold licenses, who are under regulatory restrictions, bonded, insured, hold to standards of ethics, then I'd suggest you learn what why and how they do things. :)