We bought our house with a conditional contract - well, technically, my mum bought it, as she was selling her house to move into a flat anyway. Fortunately she got an offer on her house straight away - it wasn't at auction - but it got a bit tricky when her buyer had to delay settlement which in turn delayed our settlement (and our seller's settlement too, and they had bridging finance). In the end it worked out though. From experience, prepare for the seller to not be as flexible with accepting lower offers if you want a conditional contract, and have bridging finance lined up because they do have an opt out - if they get a non-conditional offer they require settlement within (I think) forty-eight hours or they can opt out.