They all use a SIM card and I've not found a SIM card yet that won't expire if not used for a period.
That is true. But if you get the tracker to send a text message back does that not keep the sim active or does it have to be a phonecall? You could do this once a month or so to keep the credit balance alive.
Also, the Ebay trackers send position reports at varying intervals, which costs you credit. However, I can't find out of you can turn off position reports when you don't need them and turn them back on when you do. If you can't stop the position reports totally, that could be a nice little earner for the service provider!
I found this PDF manual for the product in my link:
http://www.zhyichina.com/en/GPSTracking/GPSTrackerManualTK-104.pdf
The English is pretty poor but section 6.3 seems to indicate that you can turn this feature off.
I've toyed with the idea of purchasing one of these devices but haven't come across anyone who has fitted one and has had first hand experience. Hope this helps. If you do buy one let me know how it goes.
Update: Found this thread which may be useful:
http://forums.mercedesclub.org.uk/showthread.php?t=61285
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