Ex-showroom / display electric oven warning!
It is common practice for cooling fans to be disconnected when on display in showrooms etc. This is not the one inside the oven but in the space above the oven cavity, hidden away that keeps the electronic panels & controls cool. They usually continue to operate after the oven has been turned off (to "standby") so if it's working, you're lucky!
If you're not aware of this, the oven may appear to work fine initially but it will probably begin to malfunction over time, cutting out or intermittently switching off during use, eventually ceasing to function due to overheating of the electronics.
Due to this "modification", the supplier / manufacturer warranty is almost certainly to have been voided. An engineer will easily spot the disconnections when they come to fix your "under warranty" oven. For example, £300 for an electronic control panel board (just one of 3 circuit boards in a single oven)...plus the call-out charge when you probably have no genuine receipt! :O
I recommend that you get confirmation that the oven is in original working order with the cooling fans connected before bidding. It's quite likely that the seller won't even know about any of this and will be selling ex-display, unused with "full warranty" but you may find out otherwise in time.