There are various ways one can sell opals, To friends, to a dealer locally, online or by auction.
If selling to friends or locally of course you can show the opal and let them examine it and negotiate a price directly.
Selling by auction, such as on ebay for example, or to a deal online is somewhat different. You cannot show the opal so will have to show pictures of the stone and describe the stone in great detail. One needs to be very accurate when doing this to avoid a buyer coming back and saying it was not as described. With the picture one should mention that this is a representation and a pictures does not do justification to the stone.
The important points when selling opals are:
Ensure you describe the opal exactly. Possibly the same description you received when you first bought the opal.
Provide a picture of the certificate of authenticity you received with the opal and state that the certificate comes with the opal.
If selling by auction it would probably pay to have a reserve price on the opal to ensure you do not sell it at a loss if there is little bidding on the stone.
You might be likely to get more if you sell to a private individual or even on auction than you would from a dealer. However the dealer is more likely to be trustworthy as they are there for the business and not likely to run off with the goods.
Of course you need to establish the bona fides of any buyer if you are selling privately.
This applies to opal jewellery also. You might have an opal in a ring or a pendant or even a matching pair of earrings. It is often a good idea to get them valued before you sell them so you can then provide a certificate of value.