There are several options open to you if you want to sell an antique or collectable item.

Sell to a dealer
Sell through a dealer on commission
Selling at auction
Sell privately

Sell To A Dealer
Seek a dealer whose stock includes similar items to the one you wish to sell, and consider trying more than one dealer. If you need help in finding a suitable dealer seek advice from a trade association. They will recommend a dealer near you and who they know to be reputable.

If a dealer is interested in what you wish to sell, he will be happy to
travel a reasonable distance to see the items. Do not allow anyone into your home that has not made a prior appointment with you. Better still make an appointment and take the items to the dealer.

Do not expect the price you are offered to be as high as the price at which the dealer will sell your piece - like everyone else he too has to make a profit to live. Prices you are offered may vary considerably. This is not dishonest, but is likely to reflect different opinions as to what the dealer believes he can sell the item for. A dealer will pay you immediately, without any deductions.

Sell Through A Dealer On Commission
Some dealers will accept a piece for a limited time to sell on your
behalf, taking a percentage of the sale price as a fee.

If you choose to sell in this option you should agree in writing the
following terms: the minimum selling price, percentage of the sale price to be retained by the dealer, who will insure the piece, and conditions under which the agreement can be terminated.
 
Selling At Auction
The advantage of selling at auction is that your item is exposed to a number of potential buyers at the same time. One of the disadvantages is that this can produce a very low price if there is not much interest in your item, as well as a good price if there are competing bids.

Remember that if you sell at auction you should agree a reserve price below which you do not wish to sell - but don't forget that if it fails to sell you still have to pay a charge.

You must wait for a suitable auction before you can attempt to sell your piece. When you do sell then you must pay the auctioneer a commission - typically 15% of the sale price, plus VAT. There will also be charges for insurance, any catalogue photograph, and recently some auction houses are also deducting a handling charge from your sale proceeds. Therefore do not be disappointed if the money you eventually receive is a lot less than you expected. You will not receive payment from the sale until some time after the item has been sold.

Alternatively you can consider selling on the internet via the well-known internet auction houses. Ebay and QXL are the most graphically spread and hence the most popular auction houses but there are also a number of other more specialist sites you can consider.

Sell Privately
You can place an advertisement in a local or national paper or a
specialist magazine. However never include an address, and think carefully before using your telephone number: a box number is preferable.

General
After you have considered the above, decide which option best suits your particular needs.