Julie can you see the area between the pearls, around the knots, does it look slightly discolored? It could be just a trick of the light, the angle of the photo or it coud mean the pearls need re-threading.
Natural Pearls are very rare and valuable, and consider yourself lucky indeed if you get to see a native pearl in a lifetime. YOur pearls under enquiry look quite uniform in size, suggesting to me they could be imitation pearls or cultured pearls. Even with an 18 carat gold clasp I've learnt not to leap to fine gem quality pearls too quickly. You have to examine the pearls! With a hand lens examine the drill hole for evidence of plastic or synthetic material.
Organic Pearls feel "gritty" if you run them along the enamel of your teeth. Regarding the marks on the clasp, there are so many arrow marks. Hopeless to discuss arrow marks on jewelry without a photo of the mark. Sorry.
The type of pearl, the condition of the pearls, (it might be worthwhile re-threading if you can determine they are good quality pearls) the color of the pearls and basically how keen you are to sell them, will determine the price.
