Auctioneer not sure but wants deposit?

We have an offer on a house our offer is around 75% of the asking price.
Auctioneer said they are ok to sell at this price in principle and the seller has had meetings with his bank and they are 99% sure the bank will settle the mortgage at the price we are going to buy. From what I know the sellers owes the bank a good more than our asking price again our offer probably only covers 75% of his mortgage from what I can gather.
Our offer is our maximum mortgage approval.
The Auctioneer wants us to put on a booking deposit tomorrow I have emailed him my concerns this evening. I don't want to end up deposit paid and contracts signed. And then his bank say they not willing to settle with him at our buying price.
Do you think I should get some guarantees before i place the deposit?
Could a Auctioneer take a booking deposit known he might not be able to sell to us?

The booking deposit has no legal signifcance. If you choose to pull out of the sale, then you can and you will get your deposit back. If they pull out of the sale, they will give you your money back.

They may have a better case with the bank if the solicitor confirms that a deposit has been paid.

When you get the contracts you need to make a decision as to whether they can complete or not. I would be very surprised if the solicitor allowed them to produce contracts without the permission of the bank to sell. Your solicitor will probably ask to see a letter from their bank agreeing to the sale.

So, in short, don't worry now. Pay the deposit and see what happens.

Paid the deposit today and all seems ok its even gone sale agreed on all the property websites within two hours of me putting deposit on. So hopefully that is good sign. I know its not a guarantee but if they werent serious I would think they would leave it online keep their options open.