I have a Conn 10D from when I played in College. Searched and searched to find out about how old it is because I want to sell it and I want to give accurate info. The serial number is unique as I cant find any of the like on the Conn website or elsewhere. The serial number is 5 595127. I bought it used in 2001. I think it is 90's but I want to know for sure. It is two tone but mostly yellow brass with some silver on the main tuning slides and the valve chambers/triggers. As far as the condition goes it has some minor dings and shows some wear. Should I list as "fair" or "good".

I think the single spaced digit number system started in 2000.
Regarding condition, if you have to choose between fair, good, excellent etc. I would pick the higher rating of fair or good if these are results of normal wear. If you don't have to pick a category then just write what you wrote. If you do have to pick a condition level then add the description in your narative.

What is the value then of a year 2000 conn 10d? What is a fair asking price?

Well, it's not Elkhart by the way.
Most Businesses will price a Conn 10D in Good condition with minor normal wear on the lacquer for $2800. Private parties often list a similar instrument for $2500. If the valves are fast and clean and compress well this is a good price although the horn will sit around for awhile before being bought. Deduct accordingly for dings and such. And deduct according to how quickly you want to sell the horn.