If you review the NOD, and do your Due Diligence, you will probably find in most cases, the person owes more than the property's AS-IS value and in again, most circumstances has other liens, judgments and tax problems in addition to just a problem with a mortgage or 2 or 3.
Sure you can buy, but closer attention must be paid to what your buying. Attempting to do most preforeclosures as traditional real estate sales would require the seller to bring a large amount of money to the closing... something that is not really practical.
In most pre-foreclosure situations, you will need more than a traditional real estate approach to the problem. You will have to work a loss mitigation with all the defendants... all the lien holders, 2nd & 3rd mortgage holders, the tax men, state, federal and local and anyone with a recorded judgment against the property or owners.
These are not impossible task... if you know what your doing. However if you mess up and miss something, you would have a real mess on your hands and a very angry client / customer, defaulted seller and any number of other parties wanting satisfaction and looking at you.
Learn how before you blindly jump in, or only do it post foreclosure when the REO sale is more like a traditional real estate transaction.
Also make sure .... your Broker knows what your doing... or you may find your not covered by the companies E&O insurance.