Make absolutely sure that you've gathered all the information you can. Don't be surprised if gathering the information takes you several months. Above all, don't rush into the decision until you've explored every nook and cranny of the business.
Make sure that you show the information you've gathered to your lawyer and to your accountant. Ask them if they think the purchase is a good idea.
Make sure that you fully understand the true reason why the current owner wants to sell. If the business hasn't been doing well, you should know precisely what the problem is and how you can fix it. Don't buy a business on the vague hope that somehow you'll magically turn it around.
Make sure that your decision is from the head as well as the heart. Don't buy a business just because you've fallen in love with the idea of being your own boss or because you really like the building where the business is located. Don't buy it unless you are reasonably sure that you can make some money from it.
Make sure that you know how to run the business you want to buy. If you don't, take the time to learn more about it or make arrangements for the current owner to stick around after the sale to show you the ropes. If the business is sold to someone else in the meantime, you'll still be better off than you would be if you bought a business you didn't fully understand.


Conclusion

Buying a business is a complicated, emotional process. By avoiding these costly mistakes, you can prevent turning your dream into a nightmare.