Which auction house is it?
Most will have a minimum amount, so that if you try to bid €50, it will answer back"minimum bid is €100"
I see no point in showing interest ahead of the auction unless you won't be able to bid live.
Most of them have a maximum bid. Say you bid €500 and the last bid before yours was €200, it will be knocked down to you for €220.
Not sure I would trust most of the auction houses. If they know that you will bid up to €500, they will probably take bids off the wall.
Best thing is to bid live.
Watch what happens to the prior lots.
They may start off at €150 and when no one bids that, then they reduce the asking price to €100.
And if no one else bids, you can usually buy it afterwards.
Brendan