Firstly, let's deal with the question of how much money you can make. Will you make a million dollars? No. Only very high profile offline authors make this sort of money. Is it possible to make $80- $100k per year? The answer is yes but like anything it will require some hard work and a dash of entrepreneurial spirit. In fact, to be truly successful you will need to spend nearly as much time marketing yourself as you will be creating content.
It also depends on your agenda. If you're looking to make a full time income then those figures are within your reach. If however, you're a student for instance and therefore do not have as much time at your disposal, then a $20k per annum secondary income is a more likely scenario.
Do you have to be a good writer to succeed online? No. In fact being a good writer may actually be a hindrance: simply for the fact that your work may not be appreciated and command the price that you feel it's worth. The nature of the market that you will be dealing with is one that is after fast and relatively cheap content. They're not after high quality writing so don't try and be
Shakespeare. All you really need is a decent grasp of spelling and punctuation and be able to write in a conversational tone. Easy!
Do you need to have a subject specialty? Once again the answer is no and if you do specialise then you may be severely denting your income prospects. The secret here is to not put all your eggs in one basket. You want to have multiple sources of income: this means not only a multiplicity of subjects but also a multiplicity off avenues to get your writing out there. Bidding at auction sites is certainly one way of getting the dollars in but it does not preclude creating monetised blogs or writing and submitting articles with embedded links.
Quote of the Day: "The difference between a boss and a leader: A boss says, 'Go!' A leader says, 'Let's go!'"—E. M. Kelly