Try having a look at our gender identification thread in the pinned topics area here. There is no reason for you to rely on the LFS identifying the gender of your common livebearers. Once you learn to tell them apart, it will take you all of 10 seconds to decide the gender of any common livebearer and no more than a minute or two to determine the gender of less common fish like goodeids.
A breeding of mollies can be exciting if you take the right attitude of enjoying it rather than dreading it. I have bred any number of livebearers over the years and have always found the experience rewarding. If I choose to raise some fry from a particular species, I look to where I will sell most of the resulting fry. In my case that is usually a club fish auction, but in the past I have sold good quality fish to my LFS as well. When selling to a LFS, expect to only get some credit towards fish food or something similar. It will help you maintain your fish without the expenses of buying fish food which is a good thing for you and it will give the LFS some hard to find top quality fish to offer for sale. Everyone wins in this situation.
When selling through a club auction as I do, expect to give a portion of the sale proceeds to the club, auctions are fund raisers for clubs after all. In my most recent auction, just last weekend, I spent more money than I got through sales of my fish so my net is negative, but I have a nice piece of driftwood, nice decorative fish for my community tank and some new plants, along with a species of livebearer that is rare and I have never had before. You could say that this auction cost me money, most do, but the fish I sold offset most of the cost of me getting a new species of fish along with new decor. If I go to my LFS, any rare species of fish will cost me a pretty penny and things like unusual plants and decorative wood are even more dear. I figure that I got my new things almost free on a net basis and my fish room is now better for the acquisitions I made.
