Why you should not plug these things in:
First reason; compared to this day's standards in wiring - old valve amps are dangerous. They have voltages running around inside them that can kill you. A lot of them are not earthed. You don't know what the history of the amp is and why it ended-up where it is - there might be parts missing or parts damaged or a mouse might have got inside and chewed-up a few wires or dislodged a component or two.... you simply don't know.
Second reason: I am not trying to trick anyone, you can cause serious damage to an amp simply by plugging it into the mains. If you don't have the correct speakers to connect to it, and all impedence settings match, then you will very quickly burn-up the output transformer and do damage to the valves. The valves can sometimes be replaced but you will find it next to impossible to locate the correct transformer as a spare part. Modern generic replacements that don't perform quite as well are still very costly.
Third reason: potential buyers don't like it, so you will get less bids than if you had resisted the temptation to power-up.
If you have found what looks to you like an interesting item then my advice is this. Go and check the internet to see if there is any information on it first. As a starting point you can look on the advanced search option of ebay to find out if similar items have been sold here, (just type in your keywords and click on the box which is marked 'search completed listings only' - this will tell you if an amp like yours has sold recently and the price it achieved). If the amp looks like it will be worthwhile listing then find a local repair shop and take it along to get checked over by a qualified technician first. Most music/instrument shops can put you in touch with somebody who does repairs. The repair man will be able to tell you if the amp is working or needs some attention. Get a quote. Then you can either go ahead and get it repaired, or simply list the item on ebay stating what needs to be done and the likely cost of the repair. Being honest will get you a lot more bids, even if you say 'there is a problem with the item'. In the end everybody is happy and you save that vintage piece of amplifier gear from almost certain destruction.
remember- don't plug it in!