Selling Mercury is the worst way to start : Discoverer is just a Scout, so it is virtually useless in combat (i eat Scout class ships for breakfast), and Mercury is the best way to start trading and hence making money.
Here is how I started (recommend starting over as humble merchant): Get in the Mercury, and put your Discoverer in Argon Shipyard in Argon Prime. Head South Gate to Home of Light, and from there South again to Ore Belt. This is the easiest way to start up: buy Ore in one of the asteroid mines (should be 2 over there, try both to see which one has it cheaper, it all depends on how much it has in magazines (aim for C50 per ore)), and head back to Argon Prime. Now there are two factories to the left of the south gate. So as soon as you warp in, go to the factory on your left. Dock and see how much they pay for ore. If youre lucky, they will be running out pretty soon. no ore in magazines = they pay 205 per ore. Pretty good deal, you just made 400% of money invested.
Alternatively, you can try to buy and sell Cahoona stakes. Buy at the factories for cheap, sell at argon free trading station (problem is, they eventually stop buying those). As soon as you accumulate some money (say around 20k), go to Home of Light and buy trading extension in Terracorp HQ (it will allow you to check prices without having to dock in a station). Also, having any of your ships (or satellites) in system lets you access all prices from the map menu, even if you are in different sector. BUt you need the trading extension. Also, upgrade your boost, cargo space, and manuverability to maximum asap. This will allow you to trade faster and outrace competition.
You should make a pretty decent money with this ore business. From Home Of Light go East to discover the Red Light sector. there should be couple more factories that use ore. So now you have 3 potential systems to trade ore into.
Finally, to automize your Mercury, you need to first get the pilot to level 8, and buy a trading software MK3 in Home Of Light's Terracorp HQ. It costs ~520k though, so there is a good few hours of manual trading before you :)