The limit is 800k, or it was last time I checked. People usually either ask for that much in NP plus a stabily priced ETS or two or whatever you will be using the NP from the sale to buy, or (and more commonly) they put the items in trades and ask for offers to be neomailed for a private auction. If someone is interested at the right price you take the item out of the trading post and stick it in auctions for just under the price you are wanting with the mimiumum bid taking you up to your preferred selling price.
Hope that helps somewhat x)
Edit: ack posted at same time, hehe
