It's no different than if you were building any other website with SEO in mind, I'm no expert by any means and wordpress makes it pretty easy to make a search engine friendly website. Just make sure to have good titles tags that are relevant to the auctions being listed, and a good domain with a popular keyword in it.
Also make sure that you've got decent visitor tracking software installed so that once you're indexed you can pay attention to what keywords are bringing you traffic and improve on them. I found that I was getting visits for a lot of keywords that I didn't even target, so I may add those to the sites content in strategic places and watch the SERPs to see if it gives me a little boost or not.
One thing that I wasn't sure about was if the the search engines will pull keywords from the eBay listings themselves, I found out that they actually do, and long tail keywords have been a pretty big source of traffic for 2 of my sites because of the search engines getting extremely relevant keywords straight from the listings.
Also make sure that you've got decent visitor tracking software installed so that once you're indexed you can pay attention to what keywords are bringing you traffic and improve on them. I found that I was getting visits for a lot of keywords that I didn't even target, so I may add those to the sites content in strategic places and watch the SERPs to see if it gives me a little boost or not.
One thing that I wasn't sure about was if the the search engines will pull keywords from the eBay listings themselves, I found out that they actually do, and long tail keywords have been a pretty big source of traffic for 2 of my sites because of the search engines getting extremely relevant keywords straight from the listings.