It sounds like you are confusing a Buy It Now price with a Reserve. They are not the same. A Buy It Now price gives the first bidder the opportunity to either bid or buy the item for the Buy It Now price without bidding. If you had a Buy It Now price and not a reserve, the Buy It Now price disappears once a bid is placed.
There is additional fee for Reserve listings. There are no reserves on Ebay Australia other than in Ebay motors.
You must sell to the buyer once they pay or you will get negative feedback.
If you did not want $100 for the item, you should have listed it for the least amount you were willing to take for it.
By the way, are you aware you posted your question on ebay Canada?
**********************************************************************************
Non payers should get unpaid item strikes and be added to block bidders lists. Period.