Help me buy a bike seat post I know that my bike has a 25.4 mm diameter seat post. So can I purchase any old 25.4 mm diameter seat post from eBay and expect it work with my existing bike seat? Here are some pictures of my existing seat post. 

It appears that my bike seat has a little part that slips over the seat post .... I'm assuming that all I need to do is loosen the hex nuts on either side and the bike seat will let go of the old seat post and in the new one goes, right? 

ETA: will this seat post work? Thinking of pulling the trigger on this one. 

http://www.ebay.com/itm/XLC-Bike-Bicycle-Seatpost-with-Clamp-350mm-NEW-25-4-mm-Black-/221504900980?pt=US_Seatposts&amp;hash=item3392b88f74
Nearly. The diameter does need to match, but you also need to consider the materials -- metal needs grease, carbon fiber doesn't, so combining the materials in the frame and seatpost shouldn't be done. 

Also, note that different seat posts have different levels of adjustability - saddle angle and position, height (which connects to minimum/maximum insertion distance) to name some, so your seatpost will need to be adjustable enough to get a good fit for you.

As for saddles fitting on the seat post, most saddles will work with most seatposts as the rail width on the saddles is essentially standard. 

So, I'd say make sure your seat post is metal if your bike is metal, carbon fiber if your bike is carbon fiber. Then, pick something with the same diameter and enough adjustability so your bike can fit on it (for most people, even a basic seat post like the Bontrager SSR seat post will work fine, though for nice saddles like Brooks, you will want something with a bit more adjustability).