Proof of funding

The fact that you have to sell to buy puts you at the bottom of the preferred bidder scale, so if you like the property, bring whatever you need to show you have the finance to buy it, mortgage approval and deposit should do it. They just want to know that you are not wasting their time.

Apart from telling the op to jog on.

Anyone who would actually do this is an absolute dirtbag and one of the many reasons we need proper regulation around the bidding process.

If you're not prepared, approved and have the relevant documentation needed to enter a bid in good faith, then you're at best a time-wasting dreamer and at worst the kind of cretin that's out there driving up prices for people looking to put a roof over their heads.

Most decent estate agents will see you a mile off and blackball you when you inevitably get found out should you go sale agreed.

And then someone will be complaining about "false bids" and wondering how an agent could allow someone bid 350k without checking if they are genuine

Other than report you for deception or using a false instrument. Stupid suggestion. OP, you’ll need to have made some progress with the sale of your own property to be taken seriously.