Legal Question in Business Law in California

I assume that by "buy a iphone app" you intend to buy and operate a business. If another company owns the trademark you want to use for your business, there's a serious potential problem. I'd suggest contacting the trademark owner to make an initial inquiry; it may be willing to transfer the trademark to you or license you to use it. I'd also bring this problem to the attention of the seller of the app; they may have a satisfactory explanation. In any case, the registration of the name to another party is a potential deal-killer if indeed you are buying the app to run as a business.

The test is whether there is a "likelihood of confusion" between your app business and the trademark owner's business. If the registered trademark's owner is in a totally different industry, and uses readily distinguishable marks/images/graphics from yours, you would most likely not have a problem. If the registered owner is in the app, mobile device, software or other related field, you absolutely would be in for legal trouble down the road.