According to HotRdd :
I'm about to start the replacement of an old deck and was wondering if you
guys can answer a few questions for me.

1) I've reclaimed some old and heavy 10" x 10" posts from a barn that I'm
hoping to use to give the deck some character, does anyone see any problems
using this structurally?

Not structurally, no, but some posts (eg: hickory) will _not_ stand
up to weather very well, and given their age, they may go quite fast.
Now, if they're cedar, they'd should last a long time.

If you can identify what kind of wood they were, it might help.

2) What type of sealer should I use to treat this old lumber.Recommendations

Just about any, but, don't bury the posts.

3) I need to bury some posts this weekend and I'm not sure what the best
method is. Do you get a concrete tube and fill it full of concrete and then
put the post on top or do you encase the post in the concrete? WHY?

You can do it many different ways. The main things to remember:

1) If the end of the post is underground, make sure the last
few inches are in gravel, _not_ concrete. Eg: dig hole, add
a few inches of gravel, drop in post, add an inch or two more
of gravel, _then_ fill with concrete.

2) If you're using an auger to drill the holes, and the posts
ends are underground, you really don't need sonotube underground.
You only need a bit of forming to keep the top of the concrete
slightly above ground level, sloping away from the post.

3) If the posts are going to rest on top of the concrete, cast-in
proper saddle brackets (above dirt level), and the deck needs
to be supported from the side so that it doesn't sway.

4) Should I stick with treated lumber for the posts?

I probably would, without being a bit more sure of the weather
resistance of the barn posts. If you have to bury the posts,
_don't_ use the old barn posts unless they're ceder.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.