Yes, I have been selling some for sure. My original intent was to have the legs available for customers so that they could buy a slab from me, buy the legs, and make their own bench. It is a way to help me sell slabs. However, some customers have seen a bench that I have made and want me to make them one.
The key is to use a rally nice, unique one-of-a-kind, slab with a lot of character and then, do a very good job of finishing the wood so that potential buyers see it as more of a fine piece of furniture rather than just some wood with legs screwed together. From start to finish, it takes me about 5 hours total to shape the slab, sand it through 220 grit to achieve a perfect surface, attach the legs, and apply at least 4 coats of 100% pure tung oil mixed 50:50 with 100% mineral spirits.
So, figure the cost of a pair of legs shipped to you (if you buy more than 5 legs, there is a discount). Then add the cost/value of the wood, then figure the value of about 5 hours of your time, and then figure about $30 in tung oil, mineral spirits, sanding pads, etc. Whatever all that comes up to, that will be the minimum that you would need to make it worth your time and investment. You could add some for the "artistic value", and you could add some for "profit and return on your investment". This is going to put you somewhere in the $500 - $750 range for a very nicely finished bench, depending on how much you value your time, and you will have to determine if your market will bear that cost.
Den Scoling made a great point in the thread about displaying slabs. The bench slabs need to be dried and sterilized to make a fine piece that will remain flat and true and without insects. Nobody wants a coffee table or bench that wobbles because the wood was not dry, warps, and then the legs will not sit flat on the floor. The slabs have to be very well prepared. The vast majority of slabs that are for sale out there have not been prepared this way.