I wouldn't worry.How is this any different from astrology? Do customers of psychics sue them for telling them things they don't want to know?
Do people who buy plots on the moon sue if two of them end up with the same address?
Go ahead and sell paternity testing, or testing for certain cancer genes or GMO food.
Let people know that you are cheap because you have a higher error rate than the big guys.
Everyone signs the disclosure and all the risks are in large print, and you let them know the results won't stand up in court.
You aren't practicing medicine, there is no malpractice. Especially if you're testing GMO food.
Horoscopes have the same ethical issues, and lots of people make money that way.
You could test homeopathic remedies and always tell the customer "No, that is not 100% pure water" and always be right. Charge the believers more -- you already know they're gullible.
If you are testing for patented genes, or using patented techniques, licensing could be an issue. So don't. Or get the license, it may not be a big part of your operating expenses.
How about a service, call it "Own a Clone", that copies one or more of your genes, and sells a small vial of the replicated DNA on a necklace chain? Copy enough SNPs and the result is likely to be unique to that individual. A gift for Valentine's Day, or to exchange at a wedding. Put a vial of Fido's cloned DNA on the mantlepiece, after he's gone. Or skip the vial, dab some of the DNA on a postcard, it will probably last longer in the dry state. Dozens of science magazines would give you free publicity by writing articles about your lab.
How about paying celebrities for spit to amplify? Sell postcards of Bieber DNA or Jagger DNA. Or Watson or Venter.
Ethical minefields make for great publicity. How about selling vials of DNA from a hundred random people who sign releases, and market it as something to wipe all over your murder weapon before leaving the scene of the crime? With a good enough instruction page, you can sell rocks to people to use as pets.
For that matter, people on Death Row might sell you their DNA for cigarette money. Own a dab of Jeffrey Dahmer DNA, and a certificate of authenticity. The creepier it makes me feel, the more certain I am that it will make money.