I think it would probably be a good idea not to trust TECHSHARE as he seems to be lacking good judgment even though he does have an extensive trading history. I personally would never insist on a single person to act as escrow, but rather would suggest three potential escrows if my trading partner does not trust me enough to send first to me (and vice versa), and let my trading partner decide. Only if my trading partner is apethatic about which escrow is used will I suggest my preferred escrow.
Some people do not trust particular escrows, sometimes for good reasons, sometimes for bad reasons, and sometimes because they simply do not know the escrow. I would personally find it suspicious if someone was demanding to use a specific escrow and would back out of the deal when others are suggested.
I did not want to use escrow. End of story.
If this is the case then it is probably advisable to not trade with you.
As a general rule, even if you would trust someone sufficiently to send first to them, you probably should not trust them with your money/property if they are outright refusing escrow because it is much harder to detect a pending exit scam, in fact refusing to accept escrow (and backing out of a trade once escrow is mentioned) is one indicator of an exit scam in progress. The fact that TECHSHARE's trading threads are all locked also makes it difficult to detect pending trouble. This may or may not be the case in with TECHSHARE.
TECHSHARE is also egocentric and tends to be vindictive, so if you get on his bad side there is a chance that you will get a wall of text instead of the money you are owed. Anyway, sorry to the OP for the off topic writing, TECHSHARE likes to try to derail threads, and has a long history of doing so.
Anyway, back on topic, I might suggest that you find out how much a pawn shop (and/or a jewelry store) is willing to pay for your silver (and the diamond), as that would probably be a good starting point for potential buyers, and if you were willing to accept 70% of the spot value of the chain, then a pawn shop and/or jewelry store might be willing to offer you an amount that would work for you!