I've done a little buying and selling on eBay. Almost none of my transactions have been HP related. I buy very few because I've had good luck getting them for much less elsewhere and I've sold none because... well mostly because I've just sold very few HPs period :-) I also feel a little too "connected" to the HP community to use eBay. That's hard to explain - just a personal feeling. All of the items I've dealt with have been "collectible" however.
I started eBay as a buyer. I learned that prices are generally very high but you can find something good now and then. One of the best ways to get a good price is to paw through all the listings rather than searching. Good eBay prices happen due to misidentification and misspelling. This takes time - it's the old work/reward thing. Some people may think I'm ripping people off due to their mistakes or ignorance but all of these items still get a bid or two so they're going to sell cheaply to someone. It might as well be me.
Other times you can get a reasonably good price if you follow the eBay cycles. I've seen several cases where the first item of its type appeared on eBay and sold for a medium price. This gets announced on the specialty forums and newsgroups. Then a few more of the same item starts to appear, and prices rise dramatically because everyone is looking. Then bunches come out of the woodwork, and eventually the price comes back down to medium or sometimes even low. Then they often stop appearing for awhile and the cycle repeats later.
I first started selling on eBay two summers ago. I realized I needed to make some room in my house so I was originally going to throw a bunch stuff out (don't gasp - not HP stuff!) but then I decided that I should sell at least some of it. Garage sales, flea markets and antiques dealers seemed like a lot of work so I turned to eBay.
I was really surprised at how painless it is. I often grumble that eBay is the most profitable CGI script in history, but this is really a "Why didn't I think of it???" rant. (And I know it's not CGI.) In reality, while eBay surely makes tons of money due to volume, the fees on any individual sale are extremely low. The barriers to entry are essentially non-existent. Not only was the basic listing fee $.25, but you could start your auctions on credit. You could get your first items sold before sending eBay any money.
By the end of the first day, my first batch of items were already selling above the prices I thought I would get and more than I had paid for them. By the end of the week, the prices were just AMAZING! Just ask anyone who sat near my cube at work how many exclamations they heard :-) I started all my auctions at the bottom and never used a reserve. In my experience, if your title is correct, there seems to be little risk of getting a truly low price on eBay. Besides, last time I checked, they allow a seller to make one bid on their own item. This will make bidders mad and you'll probably never need it, but if you're really nervous it's good to know that you have the ultimate option in reserve.
It's not just HPs that fetch the big bucks on eBay. The items I was selling consisted of mechanical clocks and wristwatches, movie posters, old children's toys, a vacuum tube radios etc. After I ran out of my own items, I started buying items from the local antique malls and continued to make money.
In the fall, my work got intense and moved me overseas and I got out of the eBay habit. I think, though, that I may resume it this summer. It may be a little harder now, because I've found that at my favorite antiques store, most of the "good stuff" is now kept in the office where a PC keeps tabs on their current eBay prices.
My take on eBay is that if maximum money is your goal, eBay is hard to beat for selling. For buying be very selective if price is an issue.
That said, I still encourage buying, selling and trading among the HP community. The rewards are different than selling through eBay. As I said above, when I do finally get around to selling some HPs, I probably won't do it on eBay. Also, if you sell through eBay, do feel free to post an ad here too. What makes eBay work is having the maximum number of eyes focused on a limited number of items. A few more eyes never hurt. It might even save you from selling low due to a spelling mistake.