You can utilise the Self Represented Litigant people in the Court system in Queensland, their contact numbers are here: The Self-Representation Service will most likely direct you to cases you can use in support of your claim and they are on the same page as I just linked you to. To find your way in the Court on the day, there are volunteers everywhere that you can ask questions. Also if you get stuck with the preparation of your case, you can ask the Community Legal Centres in your area for assistance, their contact numbers are also on that page.
You haven't made clear to me whether you are a business selling laptops or just a private individual who was getting rid of their laptop on eBay. I think if you were selling laptops as a business, you can lodge a claim in the Queensland Civil and Administrative Tribunal (QCAT). See everything you need to know about Consumer and Trader Disputes here. On that page there is information about which forms to use, what fees to pay etc. Otherwise if you were just a private individual selling your laptop on eBay you should lodge a claim in the Magistrates Court, I've check the jurisdictional issue so you have no problem there.
So what you need to do is fill out the claim form in whichever Court or Tribunal is right, pay the fee, then after you've done that, create your own statement of what you think occurred, attaching to it any documents that prove your evidence. As you can see from this page, you don't automatically have a right to representation from a solicitor in the QCAT anyway. But you can apply to be represented if you want to but I don't recommend it as most people in the QCAT are self-represented anyway you just have to be organised about what you want to show the member of the tribunal and your story. See this page about preparing statements in the QCAT. Don't forget to say what it is you want in your statement and if you want either the money to be returned or the laptop to be returned, express this as an alternative option. There's a lot of information online so just work through it.
If you go to the Magistrates Court (MC), here is the page about representing yourself in Court. Pay particular attention to the Practice Directions there, that will tell you what to expect from the judge in the division you are suing and what to expect in the Court room often. Don't be nervous, as even solicitors are nervous when they appear there for the first time, so you are among friends. In the MC you will not prepare a statement, you will prepare an affidavit, in this format. Again, utilise the free court resources and your community legal centre to get the cases and commentary you need for breach of contract and for tort of conversion.
What you can do to understand the tort of conversion is to ask the community legal centre or someone in the Court network to copy the pages from their text book (a torts and a contracts textbook) on the topic. Alternatively, if you visit the State Library in your State they usually have a legal section there. Borrow or just take off the reference shelf a torts textbook and a contracts textbook. You might also find the tort of conversion in a personal property textbook.
You can ask the community legal centre to direct you to cases on austlii that will help you. Here is austlii's website. You can also do your own searches in there for cases.
What you need to do when looking for cases is to read through and try and make an analogy between your situation and the facts in the case, then use the judge's decision to prove your point too.
The reason I am encouraging you to do this yourself is that for the cost of a laptop, paying someone legal fees is not worth it.
If you have a low income, you can get the community legal centre to do this for you on your behalf if you meet all of their criteria.
I hope that has helped, let me know if you need more help.