Is it illegal to buy an item from a store, then sell them in my restaurant?

As I suspected, YES, it is illegal and can subject you to disciplinary action at the SLA, you have to buy it from a licensed wholesaler. There is what's called a "three tier" system in the ABC law where wholesalers must buy from manufacturers and retailers must buy from wholesalers. It is the way the sale of alcohol could be tracked and regulated.

Generally, once you legally purchase a product you are able to, in turn, resell it. However, there are many reasons you may be prevented from doing this such as contractual terms against doing so (e.g. from wholesale contract), trademark infringement (e.g. where it appears your restaurant is sponsored by the product's manufacturer), and etc. You should contact an intellectual property attorney to discuss the specifics.

If you have a liquor license you would have no problem.

I'm not quite as quick to answer as the other respondents. While I have represented many bars and restaurants over the years, I have always stayed clear of anything involving the ABC or liquor law/liquor licenses. In my experience, there are a handful of really really competent liquor lawyers who are used by virtually every restaurant in the City. That said, I would not be surprised if the ABC Laws specify WHICH companies may act as liquor purveyors or distributors for re-sale in restaurants. I recall reading, a few months back, about a very well-known chef and restauranteur who lost his liquor license and had to close down his restaurant for several months, as a result of getting his booze from the wrong distributor. If you are in the restaurant business and sell alcohol, I can not advise you strongly enough to get a good liquor lawyer. This question should be directed to him/her.

This is not really an intellectual property law issue, it's a liquor law issue. Find out from your City Clerk who the Liquor Commissioner is and call and ask.These laws vary greatly from City to City and it is not unusual for corruption and special interests to obtain special ordinances that might trip you up. You should morally be okay, but you need to know what City Hall says on this. Call and find out.