You can try using The Official Equipment Setup Review
thread as an appraisal thread, simply state that you're trying to get an appraisal.
Additionally, as egoballas
stated you can visit the specific marker forums and there may be an Official Appraisal thread. You may be able to post your own thread, but be sure to check the rules and other threads in the sticky's to be sure this is permitted.
Lastly, it may be more time consuming but you could look through BST threads for similar set ups to see what people are listing their equipment for, and what other people are offering for it. This will generally provide you with the most accurate price.