Use both! EBay is a ready-made pool of potential customers who come to the site with their wallets open. So, if you're just starting out, I would learn the ropes on eBay first. But you can drive traffic back to your own website via eBay. Once you are established, it makes sense to build up both sites. Eventually, each will draw visitors from different places, which can only help your business as you diversify your customer base.
Creating a MAP will take no more than an hour of your time every month and will keep the lines of communication open, ensuring relationships with investors remain strong, and ultimately helping early-stage startups succeed.