Well, there's nothing illegal or immoral (or, I suppose, fattening) about saying "hello" to somebody you don't know. The key thing is to take it one step at a time. Make sure the first "hello" is cheerful, kind of butch, and certainly not suggestive in any way. (No "hello, there", smirking, or batting of the eyes.)
If the guy is straight or not interested you will find out immediately. If he responds with at least equal friendliness, you can continue with, "how ya doin" or "can you believe how hot it is" or "say, where's the best place to buy jeans," or whatever. Still being neutral and somewhat impersonal, but friendly.
By now, you'll have a pretty good idea whether a hookup is possible. If it is, you don't need to worry too much about the topic of conversation. Just keep him talking long enough to exchange more signals until you're each pretty sure.
Then you close. And here I have to admit that my technique usually fails me, so maybe someone else will have some pointers. But for sure don't give up yet. How about something like "You know, now that I think of it, I really don't feel like going grocery shopping. You want to go outside and get some air?"
If he accepts, it's now an official pickup. As soon as you get out of the mall, say, "Hey would you like to come over to my place for a beer?"
Under no circumstances invite him to a neutral place for a cup of coffee. That would be a date, and we're talking about cruising here, right? By the time he finishes the coffee, he could be out of the mood again. Make sure you get him onto your turf (or his, if he suggests it) as soon as possible.