This is my approach to selling scuba equipment. As background, I have sold over 14 housings for friends or myself, it is a hobby, not a business.
Description: Write a clear and complete list of items for sale. Describe each item, with all the good and bad points, do not leave out any bad points, be completely objective and clear. Edit your write-up carefully over several days to make sure it is objective, clear and complete, take your time, writing is difficult for all of us, it is hard work, do it with the reader in mind.
Photos: Post many clear photos, so the customer can see understand and decide in your favor.
Questions: If the client emails a question, reply immediately, clearly, honestly and completely. Not replying equal no sale.
Price: Set a reasonable sale price for this look at similar equipment on sale in eBay, Wetpixel, Scubaboard, etc. If shipping within the same country, free shipping is a nice incentive to the buyer. Make sure the cost of shipping is fair and clear to the buyer.
Payment: I use Paypal for the transaction to provide security for the buyer and myself, no problems thus far..
Shipment: Overseas shipment of equipment can be tricky, so discuss the best means with the buyer to make sure she/he receives it. I have shipped Leica cameras to buyers in Russia and Malaysia, for example, using eBay's shipping service with no problem.