Go back to the Ebay page you found the amp on,..... look at the left side of the page,.... there is a choice there to search completed auctions. do that, and look for the GREEN numbers on the items,... that means it sold,....... and if you find one like yours,.... that would be a starting place for yours.......
Ebay is very fickle,..... from week to week, and even the time of day your sale starts can have an affect on how much your item brings......
here are some KEYS for a good sale...
1. In the title, make sure you cram as much info into it as possible,....mention key words like MULLARD, or TELEFUNKEN, or whatever the model is, and watts,.... use all of the letters that you can, eliminate all spaces, commas,etc so you get more bang for the buck.....
2. make sure you use the Gallery feature, with at least 4-6 photos,.....
3. Shipping calculator inserted on the page.....
4. In the description, be sure to say whether it works or not, any mods, or changes, be as long winded as you want, but leave out personal opinions about the item,.... it can turn a buyer off if they think you are exaggerating.
Be honest, and be sure to mention it is used, and as such comes with no warranty. All these things are important if you want a great auction.
5. Set up a Paypal account, and open a checking account at your local bank.....tie the two together,.... this makes you a 'Certified' seller,.and just means you appear to be in it for a long run rather than a quick in and out.It makes skeptics feel a little better about parting with hundreds of dollars......from someone they don't know, and will probably never meet.
6. be sure to be straight about the shipping... whoever is going to bid on your item wants to be sure about receiving it promptly, and in good shape,( just realize that shipping agents are not all that careful with your packages.)pack it with PLENTY of foam, wrapping, peanuts, or however you think it will be protected best.
I am on my 3rd group of stuff with about 6-10 each time,..... mostly old tube driven stereo gear,..... wonderful sounding by the way....right now it's a few amps, an FM radio, a Yamaha recorder,..... an Oscilloscope,..an old radio, and another piece of test gear..... and some here will be very surprised to see me here, but a friend has said they were mis-treated here, and I came to have a look around and see what is up...
good luck with your amp sale!
Steve