I've just added categories to my wordpress site and some of the posts show in several of the categories. Will this cause me duplicate content problems as I want the category pages to be indexed? Also as I add more categories I'm creating more links on the page. They can't be seen to the user as I have a plugin that creates drop down categories. When I go to 'view source' though all the links are there so google will see lots of links. How can I fix the too many links problem? And should I worry about duplicate content issue?

You need to create a new post for each plane you have for sale (unless you've done this already) and the assign the category to each of the posts. Each aircraft added in your case will populate the category page however you should be sure to add unique content to the category page. A good way to do this is to add a category description at the top of the category page. If you're using wordpress then I use the 'all in one category updater' to do this. You obviously need to make sure that the category pages can be indexed by Google and then build quality links to the category page from external websites using a mix of brand, partial match keywords, exact match keywords and url links. The key here is natural variation.

I'm currently using SEO category updater plugin. I have about 10 posts per category showing snippets of each not the full page. If I write a new title, and description for each page, about around 50 words of text would that be enough for google to identify the page as being unique? The problem is I have hundreds of categories so it could be very time consuming.

Can anyone help with how I add a sub domain to webmaster tools? Also do I need to create a seperate sitemap for each sub domain?

Any help appreciated!