What hasn't been mentioned is that you need to check who really owns the trailer.
Check on this website
Personal Property Securities RegistryThe site is a PITA to use, but take the time to get the VIN other identifying ID's from the trailer and run it through, also buy the certificates if it comes back as clear title (eg not financed, or used as an asset backing a loan).
Also check on the QLD govt website for there RTA (Roads, Traffic, Licensing etc...) and see if you can look up the rego of the trailer and who it's registered to.
All of this sort of data should line up.
If there is any search data and story that doesn't line up treat it as suspect and walk away.
No amount of fancy story telling we let you keep a trailer that is later recovered as stolen, under finance etc...