I am not certain of what you are asking but let me try to help. My answers are for US tmobile only, I've no information on non-US SIM and carrier options.
In general, yes you can use a Tmobile SIM on an iPad Air. The iPad Air is unlocked, and not tied to tmobile or ATT. For example, tmobile has a 200MB per month free data plan. You can sign up for that plan by inserting a new tmobile SIM into the iPad Air and going through the sign up process.
However some tmobile plans only work with phones. The 5gig/100 minute/unlimited texts walmart plan will NOT work with the Air because the plan only works with phones and not tablets.
I don't know what data sim listings on ebay you are referring to. Can you post a link to one? T-mobile SIMs in the US are often free or very inexpensive when obtained directly from tmobile. I don't know why anyone would purchase one from ebay. If the listing said the sim was already active with a certain amount do data on it, I would not trust the listing. Again, if you had an example listing, that might help us understand what you are talking about.