Seriously first reasonable offer and its yours.
If you could include all the data about the diamonds and the ring itself that would be much better help to you as seller and potential buyers. I saw your ad and I looked because I'm post relationship lol...And I have a ring that will be going - So I'm gathering all the data on the diamond(s) and the setting. The primary diamond or diamonds should, if possible - list the 5 C's - (from memory so I will forget a few lol) - color (clear or colorless or nearly), clarity, cut (often the most important so I heard, that's why I went with better cut over carat size), carat (size), and OFC I forgot the 5th.....
Also list what the size if you didn't and very importantly - the metal of the band - silver or platinum etc. If it's silver and you know, mention if it's plated too. I'm definitely no jeweler but I can tell you that I personally paid way too much for a ring when I was young and under the influence of oxytocin and/or other hormones. This was before BTC existed; a very very bad investment looking back at it. Hell I even thought it was a bad idea when I walked into the jeweler! I went back to them around 10 years later to have that thing cleaned and ask what I could get for it. They had always cleaned it and so on so I figured they would be a good place to start. They offered me about 20% of my purchase price. Nice! So I approached a few more shops and they were all in thesame sort of range, I think the high was around 30-35%. I should have taken the money then and there and bought BTC!
Good luck. Put in as much detail as you can and you will have better luck!
PS: Just remembered the 5th C - it is "certification" - so you might not have that. I think I do, there are a few agencies that certify diamonds AFIK and it lends credence to the rock.