First determine the cost of materials amount of time it will take to make. If you are going to sell it as a indoor/outdoor table you must use material that will weather the elements. That is Cedar, Teak. Cypress or a Trex like product. I would make sure that the insert does not collect water. See how much it would cost, remember you want to get paid and ask around to neighbors what they would expect to pay. See if it makes sense to do it. Remember most decks don't have a cover. You could make a set of furniture that matches so they have a choice of pieces and styles. If you live in Ca. you could make it out of redwood. other places you can do cedar. Make a sample or 2 and have a bbq and ask your friends if they would like a custom built patio set or bar. As far as selling it online you will be selling against Walmart, Amazon, mostly made in China. You want to sell high quality, custom made, that you will personally back deliver and install.