Hi Jez,
Check the knobs, make sure they're not unduly loose/wiggly, but only do so LIGHTLY, do not use much pressure, just light finger-tips. If any knobs are wobbly they should still work perfectly well, but ideally they would need to be replaced with new potentiometers by yourself at some point, which if the TI is anything like my
Indigo was it can be a fiddly job.
Check the buttons aren't sticky/depressed. Check all the keys work, and check the travel of the two modulation wheels (preferably with a patch loaded up so you can audibly test the full travel of modulation of the wheels). Unsure of your proficiency, but if on the off-chance you are brand new to hardware synths one mod wheel is
meant to be spring-loaded and the other isn't - just in case you think one may be broken!
Check the power cable is included, and USB cable (the latter is a standard
USB-A to USB-B cable).
If you buy it, the previous own will have to de-register the product via Access support before you can re-register it for yourself via Access' site. This will need to be done in order to be able to use the VSTi aspect in your DAW. Basically it lets Access know the unit hasn't been stolen or the like.
To do this, the former owner simply needs to contact Access support and request deletion of his registry. You can then re-register it at your leisure.
Lastly, you may wish to check the OS version currently installed on the unit - the version is shown on the LCD during the boot sequence when you first turn the unit on. The latest version is 5.0.8.00. However you can update this yourself, it's no biggie, just requires a PC/Mac with internet, USB cable and also most likely you will need the product to be registred (as mentioned above).
Any Q's, let me know.
All the best,
Timo