The guy selling jewelry out of the back of a dark grey van gives good deals. Lots of “two fors”. 
Sort of related: I saw some gold cuff links that I liked at a gold/jewelry store in Alexandria, VA and asked how much they were. The guy at the counter told me to make an offer. I suspect the policy differs from store to store, and not by much.
Forty years ago my future (and current) wife and I went to the Birmingham (England) Jewellery Quarter to buy an engagement ring. There are scores of retailers there, some with huge window displays and many occupying a single small unit.
We spent all morning walking round and gawping at the displays, then we stopped for lunch and tried to reconcile ambition and financial resource.
We decided to bypass the big shops with oh-so-elegant sales staff in favour of the smaller outlets in unfashionable side streets. I think we went to about four in all and thee of them did a really good job finding the best they had within our budget.
We ended up back at our first choice. The ring is not fancy and the stone is small, but it does sparkle well and my wife has told many people about our experience since then.
We went back to the same shop for the wedding band, and five years ago the same place made wedding bands for our daughter and her husband from a gold chain I inherited from my father.
This policy looks like a great way to increase the customer’s chances of being ripped off. Let’s just think through the possible scenarios that can happen when the customer is asked to make an offer:
- The customer significantly overestimates the value of the item (including the jeweller’s mark-up). Result: The jeweller will gladly say yes and take the customer’s money.
- The customer significantly underestimates the value. Result: The jeweller will say no, and possibly make a more realistic counter-offer.
- The customer makes a realistic bid that is in line with the fair value of the item. Result: The jeweller will probably say yes.
Under scenarios 2 and 3, the result would, in most cases, be the same as under a traditional system (whereby the jeweller puts a price tag on the item). Under scenario 1, the customer overpays significantly. I don’t see how such a policy could ever be anything but to the customer’s disadvantage.
Were I the customer, I’d start by asking the purity of the gold and the mass, and then use the melting price plus some small percentage as my initial offer.
That works for items that are just plain metal, but as soon as precious stones are involved, you’re back in the information asymmetry game. As a non-expert, the odds are hopelessly stacked against you, relatve to a professional jeweller, in making a realistic price estimate. You need to take the colour, the clarity, and the cut of the stone into accoung, and then the relationship between price and weight is non-linear.
I don’t know about jewelry stores, but in Manhattan’s Diamond district you can and are almost expected to. I was able to negotiate a ring my wife really liked from $3200 down to $2400 but then to $1600 just by saying I will keep shopping.