That "prestigious" university you spoke of is Wuhan University, a place that used to be good, but is going downhill fast. Yes, they asked potential FT's to travel to Wuhan and do a demo class, however they did not offer to pay for airfares and accommodation, stingy buggers!

That's correct! Twas Wuhan University. Only a fool would fly there from abroad for a demo without recompense for travel and accommodation.
Maybe there are some foreign fools with too much money and too little common sense who have done so in the past.
Some key Chinese universities are getting too big for their boots! The English departments are headed by Chinese who think their seats of learning are prestigious. Maybe within China, but certainly not on the world stage, where they are ranked at best just average! I suppose there must be some foreign fools around that don't know this! They'll likely be only too pleased to attend demos at their own expense.

Keep things in perspective - it is never wrong to give a demo class when asked for it,

Okay, according to you, a foreigner in China 16 hours by train away from a prospective Chinese employer should waste much time and money to give a demo class, in the knowledge that there's no guarantee of a job offer, and that unqualified Chinese assessors and students will be the judges.
You can say the foreigner who agrees to do so is like a very stupid duckling that strays within biting range of a crocodile, without knowing whether the reptile is hungry or not!
By the way, one of China's most prestigious universities had recently invited foreigners, even those residing abroad, to give demos at that esteemed seat of learning without any recompense for travelling expenses! Ridiculous, but ok re what you have stated!

but it's always wrong to work for a crappy training centre, especially when asking their teachers for a deposit!

Is wrong the right word? Do you mean illegal, I don't think so. Maybe we should replace 'wrong' with ill-advised!

I think you realise by this time that I'm not a STUPID duckling!