When all else fails, they will hang up on you and not answer calls any more.

So she failed and now she's demanding.

The truth is, you've been fortunate and haven't had to deal with a difficult tenant until now, so you haven't learned the hard lessons and you failed to prepare for the inevitable nonsense that landlords have to deal with.The law gives you 30 days to deal with the deposit, but you chose to wait and hope the problem would go away.

That being said, most part time agents fail and you also have to think about your clients best interest.

Not only are they trying to watch out for there safety, but hey they are bringing up a very valid reason of up keep on your property just as if you had an appliance fail, you go fix it, but only this situation is more reason to give you more service/money made/good review if you show that you give somewhat of a care in the world if the people that are paying your bills are in good hands.

Otherwise if buyer fails to pay, seller has to incur legal expenses either to take back their ownership through deed or to assert a claim against buyers other assets (if there are any). 

I'm a newbie to all of this and I will either fail or succeed ! 

I daresay that, for conforming loan, credit is pretty much pass-fail and not hard to get where it needs to be in most cases (my doing what I mention above)... but income is what really matters, once credit is "pass" how much will your income allow you to qualify for?

If you fail to make your monthly house payments to the bank on time, that reflects on the Seller’s credit.

You're the one that will be paying the taxes, penalties, and consequences if you take this route and it fails.