I bought a really good looking one off Ebay, in Australia...
Thought I was safe buying local but they sent me one that would not turn On unless it was plugged in..
I recon it was faulty but the seller said it wasn’t..ie: ”it work good you must plug in before work”
So, I asked why the hell do you need to put in a 9 volt battery if it won’t work unless its plugged into your car..
I haven't bought another since and occasionally use my son’s (thousands of dollars worth) unit as he works for the RAC
Try and get a cheap one from your local store so you don’t get the runaround and bullshit involved all the Ebay red tape and with posting it back, if it don’t work right ..
Although you could be lucky and get a good one from Ebay, you never know....