I have a used Stella Guitar I wish to sell. What is the best way to determine its value and what, other than its physical condition determines value.

Who or what type of businesses would give me an reliable assessment ?

Might be a bit late, considering the date of your post, but I would say you should try going into various pawn shops or music shops that take used gear. Don't say that you want to sell it to them though; make up a story about how you just want an appraisal, or something like that. This might prevent them from giving you too low a price. If you do this, I would advise you put on new strings, set it up as best you can, and clean it, simply to try and make it look as good as possible beforehand.

Also, you could try joining a guitar forum online, as there are plenty of people on such sites you could ask to get information, provided you post pictures and give as many details about the guitar as you can.