Its always a bit of a pain this one.
The correct way is to cancel the rego and they should get a permit to travel, but the system is mainly designed for motor vehicles, rather than trailers. The buyer will have to get a temporary rego from the Vic end and I don't think Vic Transport will actually recognise Qld gas certificates or rego inspections, so the vehicle is completely unregistered.
Personally, I think that for you its pretty low risk. Its highly unlikely they are going to do any ram-raids with a trailer on and CTP is paid on the tow vehicle. If you are selling it and he leaves with Qld rego & your plate on, then you will have to have the gas certificate and rego inspection anyway (of course you had all they before offering it for sale as per the legislation).
Get a receipt, fill out Part B of the F3520 Vehicle Registration Transfer Application (
https://www.support.transport.qld.gov.au/qt/formsdat.nsf/forms/QF3520/$file/F3520_CFD.pdf) You can also go on-line and inform Qld Transport that you have disposed of it but at the end of the day its the buyers responsibility to sort out the transfer.
The buyer has 14 days to do that, so that should be enough time for them to get home and to mail the plate back to you and you can get some of your rego $$ back, if they don't send it back, then you have proof you sold it and, again, its the buyers problem (you will be out of pocket for the left over rego though)
https://www.qld.gov.au/transport/registration/transfer/online