The site you referenced put the value of your '77 Ambassador at between $4,000 and $7,500 if its in 'average' condition (usable without any significant or obvious flaws). This fits my impression of the market. I paid $6000 for my '75 Ambassador 15 years ago.
There aren't a lot of the Ambassadors around. The Sovereign
(31') were the common large trailer and most people went either that way or down to the Overlander (26') or smaller.
For all intents and purposes, you can use any Sovereigns in the '75 - '80 age range on the market for comparisons. The Ambassador should sell for about the same - maybe a few hundred less.
If you really want to sell yourself short ($2500 for a usable trailer), let me know! But I think you'd be doing yourself a favor to go high ($7k or $8k depedning upon condition) and make it clear you'll consider offers.
Of course, if you have a trailer that has been up to Alaska a few times, has rear frame sag, hail and rock beaten skin, lots of evidence of serious use, very old tires, and has been sitting around a lot alone and neglected, then $2500 might be generous.
There are a lot of good places to announce your 'for sale' - including this website. Good luck. Get a good value and find a good new home for your Ambassador.