I have eBay.co.uk. Should I set up a new account on eBay.com, or change my settings to include the US?

1. You should use your UK eBay account for buying and for your first listing.
2. After that, you can choose to either list and sell via your UK eBay account and target the UK market or via eBay and sell to the US market. If to the US, simply go to eBay, and use the same login credentials as you used for your UK eBay account.