Automatic Transmissions are pretty complex pieces of kit. Way outside the ability range of your average garage.
The symptoms mentioned could be an indication of neglect, or a tired box ie very high mileage, or could be as simple as a blocked trans fluid filter.
If the fluid looks like it was recently replaced, it could also be the wrong type of trans fluid was used, ( this happens a lot ) which can also generate this type of behaviour.
If you want the box checked out by a specialist transmission workshop I do know of one that is properly trained ( trained in the USA and Germany) who will know how to diagnose without guessing their way through it . Let me know and I can pass on their workshop details in Sek Kong. It should be sorted out by them within a couple of days for something simple, to about a couple of weeks for a full trans reco/overhaul .
Avoid using clowns, because you will be without a vehicle for a number of weeks if you do..
The used dealer I doubt will be of much use and if they do get involved it will likely just be a cheap patch up job, or they will expect you to pay the parts cost, which could be quite expensive on something like a transmission repair, ie a Valve body = $12,000hkd upwards etc, and they'll pay labour, which is usually lower cost. Fingers crossed it isnt a buggered torque converter...The Warranty wont be worth the paper it's written on.