After much research and a carefully worded query letter, I sold my first novel to my dream publisher myself. I also negotiated the contract myself with the aid of the Australian Society of Authors contract advice service. I have lots of experience in this type of negotiation from my pre-writing career and I don’t believe it impedes my creative work. So far (post-editing, pre-publication), I couldn’t be happier. However recently a very successful novelist told me that if I do this for my second novel I am ‘naïve’. She said it was acceptable to attempt the sale of your first novel yourself, because with no contacts or profile, finding an agent can be just as difficult as finding a publisher. For a second novel, she said, there was no excuse. To avoid being ‘ripped off’ and for help in building a career, it’s essential to use an agent. She also said that if my first novel is successful, it won’t be difficult to find one.
First up: congratulations on your first novel! That is a fantastic accomplishment. Now to the question.
There are some 'ifs' here. If your previous career was as a lawyer or something like that (you mentioned negotiating), you probably don't need an agent. If you're happy with your publishing company (not necessarily your publisher/commissioning editor, as they may leave), you probably don't need an agent. If you feel comfortable handling the negotiation side of things and you think it won't get in the way of your writing, you probably don't need an agent. This country is different to the US, where every writer needs an agent and most publishers want them to have one. Often publishers here want authors to have agents, too, but that doesn't mean they won't deal with you without one.
An agent would be useful to you if you ever find yourself disagreeing with your publisher and think you can't make your case strongly enough, or if you want to explore your options with other publishers and aren't sure how to do this. Agents are of great use to authors who don't wish to deal with the business side of things, but you seem very comfortable with that. However, it can't hurt you to talk to a few agents if you're curious - having a chat doesn't commit you to anything! Ask them what they can offer you that you're not already doing for yourself. If you meet one you really like, you might think, 'Great, I don't have to worry about all that business stuff any more and I completely trust them to take care of it', and there's your answer about whether you need one. And if instead you think, 'I can do all of this myself', you know that answer too.
As for whether a successful first novel makes it easier to find an agent: yes. Particularly if we've read it and liked it! But you'd need to have a manuscript to take to the agent - they won't sign you up with nothing to send to publishers, unless you wanted to talk to them about handling your foreign rights. And that's a different topic ...