In regards to the resale price, I believe there have been a few past threads addressing this issue, so the search function of the forum should dig some of them up to help you out.
Beyond that, I recommend looking on the forum's For Sale section and seeing what similar models in similar condition have been listed and sold for. Also look on Ebay and search for your make/model and narrow the results by "Sold Listings" to see what prices they have sold for. You can also repeat the same search process on Reverb (a legit resale site similar to Ebay but specifically geared toward musical instruments).
You can also bring it into a local music store and let them appraise it, if they offer such services.
Between all of the above, hopefully you'll be able to get a good idea of a realistic price range for your euphonium!
Willson 2900 TA-1 Euphonium - Denis Wick 4AM
Yamaha YSL-643 Trombone - Bob Reeves BrassArk 5G "Gladstone"
Yamaha YSL-8440 Trombone - Denis Wick 5BS
VMI 3301S BBb Tuba - Schilke Helleberg
Past:
York Preference 3067 Euphonium - Denis Wick 4AL
Benge 165F Trombone - Benge Marcellus
Wessex BR140 Baritone - Denis Wick 6BS
F.E. Olds Special Trombone (ca. 1941)