Buying jewellery in the Sablon - is making an offer "allowed"?

Minus 10% won't offend. Minus 40% might.

As with anything in this type of situation, it will all depend on how you ask for it, and if they think you're a genuine buyer.

If you offer 10% under the asking price and it is immediately accepted, you will spend the rest of your life wondering if an offer of 25% under would have been accepted.
If I was buying something that had been listed for 6 months I would offer at least a third under the asking price. ('I like it but it's must more than I want to pay'.) You will soon get a feeling for what the seller is expecting.
Remember, you can always offer more but, once a price has been agreed, you can't offer less. No-one ever feels insulted once they have made their sale. And no-one ever sells for less than they paid.

There are two approaches to bringing down a price. One is to offer a price, and as the others pointed out, you can't back out of it. The other is not to offer a price to offer but force the other party do so, and then you downplay whatever reduction they offer and keep pushing. Sometimes you have to make a show as if you are about to leave, but turn around a make a lower offer. Think not only in percent but also in actual amount.