Looks to be in great shape. This is an older style Sovereign with the soldered leadpipe and heavier gauge metal, Besson would change over to the Sovereign GS model the year after this horn was build. I wrote an essay not to long ago about the history of the Besson Sovereign euphonium model and the differences between several generations of manufacturing, you can read about here: http://www.dwerden.com/forum/showthr...ory-amp-review
The condition looks good considering it's nearly 30 years old. Silverplate seems to be excellent and that one dent shouldn't affect the value. Valves should be fine too for this age.
Considering that the horn is in fantastic shape comes with what seems to be the original hardcase and is pretty desirable to a lot euphonium players, I wouldn't mind asking $4500. Asking price and selling price can be different though and it all depends on the current demand. Considering it's a Sovereign and the condition you shouldn't have any trouble selling.
Music educator - Brass Instruments Enthusiast - Euphonium Player
2019 Besson Sovereign 967T-2 - Alliance DC3