Yes, you should bother!
No, you should not assume there is no patent on them; there just might be a patent. Simply, placing your logo on it would not make it different enough.
But, I would search promotional products and look for flip flops that are identical (http://www.extra-mile.com/sandals/flipflops.htm) or search for private label flip flops (http://www.customlogoflipflops.com/private_label.html). This way, you should not have to worry about your logo infringing.
You stated, "I have an idea about a way to sell flip flops," it sounds like you are interested in the marketing of flip flops and not the creation of your own brand. Either way, look into private labeling.
Hope this helps.