George III 1788 Gold Half Guinea, how much is worth and where to sell?

As of 2006, the value quoted in one website (see link #1, below) for that coin was:
鈥?UK拢100 for coin in "fine" condition,
鈥?UK拢200 for coin in "very fine" condition,
鈥?UK拢500 for coin in "extremely fine" condition.

The definition of conditions (see link #2, below) is as follows:

鈥?Fine (F): Worn over whole area, but only the highest spots are worn completely through. (US: VF-20)

鈥?Very Fine (VF): Detail clear, but obvious evidence of very limited circulation. High spots worn but detail remains. Traces of mint lustre may linger amongst the letters of the inscription. (US: EF-40)

鈥?Extremely Fine (EF): Slight wear on high spots on close inspection, and all other detail clear and sharp. Much mint lustre may remain. May appear uncirculated to the naked eye. (US: MS-60)

As for where to sell it, that would depend on where you live. Check out Google for dealers of valuable coins in your vicinity or doing business by mail and over the net.

By the way, the "bullion" value of the coin, i.e., the gold itself was about UK拢50 in 2006. It is probably a bit higher now.