I've been buying and selling on eBay for over 20 years. I recently sold an unopened Gameboy for $425 and a pair of Dragon 1/72 scale Shermans for over $85 for the pair (two separate auctions sold to the same guy).
I'm not an expert, but I have noticed time of day of the auction ending time helps if it ends Sunday evening when bidders are more likely to be home.
I start auctions low and let demand set the price. Do some homework with a quick search of the items to see what they are being listed for and don't forget to search for recent completed auctions to see what they sold for or if they sold at all.
For my example, I noticed that the two Shermans I listed had recently sold for $25 and $45 each. I started both actions around the $10 mark. They sold for $33 and $52 respectively with 5 and 10 bids per.
For many of my 1/72 scale aircraft, there wasn't a lot of demand so I lumped them together in similar lots (WW2 German, WW2 US, WW2 Japanese, UK, etc.). For these kits, I wasn't trying to get real value money for them. I just wanted them gone in the easiest way possible. I also bunched them into amounts that would fit into a standard sized box. Likewise, I started these at $9.99.
Like Bish says, starting low is key. Ask for a reasonable price and most will pass the auction by because they are looking for a bargain. But if the item is in demand, bidders will raise the price beyond what a reasonable price should be.
Offer combined shipping. That way if one person is interested in several of your auctions, it motivates them to buy together. It's also easier sending several items to one guy than multiple items to different buyers.
I sell enough so that I wait for eBay selling incentives. When I sold the three items I mentioned, eBay had a 25% off final value auction fees for items listed and sold over $25 during the promotional period. It saved me over $10 in fees when the Game Boy sold.
My internet hiccupped (or FSM's site) during this reply so my last paragraphs got lost. Any way, descriptions are very important as well as photographs. A seller who says, "it looks complete, but I'm not sure" is going to lose bids that a seller who says they have inventoried the kit and it is complete.
If I am selling an opened kit, I will bag any loose parts along with the kit decals and clear parts. If there are a number of loose parts, I will add a sticky note to the bag inventorying the part numbers on the note.
Adding photos is simple if you have a smart phone and download the eBay app. I type up the auction on my laptop and then use the option to add photos from my smart phone. It then sends my phone a message and I can either upload a photo already taken or take them then.