Trailer decking.

[Billyworld]

You are asking for opinions, so I'm going to give you mine. I mean no disrespect to anyone, i only wish to help. I live in GA so that's my frame of reference, but i would think the following applies most areas. I know wood pretty well. You're not changing the wood bc it's free, we'll start there.
Untreated wood is not for outside use, so it has to be treated. Others have mentioned oil. That may be free to you as well. If not, just buy a gallon of wood preservative, but you have to really soak them with it, especially the ends. You say you have pine, but it may be spruce. If so, i wouldn't waste my time and money.
2x4's mean more surface area is exposed, and everywhere it contacts other surfaces, it will harbor moisture and begin to rot right away. When it comes to fasteners, use cheap galvanized screws. You are going to need a lot of them too. You will be redoing this in a few years, so don't waste money on fancy ones. I redecked mine about 15 years ago with pressure treated, and it's still there. When i bought it new, the wood was untreated, and after a few years i couldn't walk on it. Only you can decide what is really cheaper. Please remember, I'm only giving my opinion. Good luck with whatever route you choose. Peace

 

[blazinzuk]

The more info I have the better.

Thanks all for the advice.

But it looks like this is gonna end up being a cheap and easy deal. As someone still hasnt bought his own trailer yet. So I basically have Tuesday to get this all done.

But I do intend on maybe revisiting this thread in the spring.

Unfortunately for now time is against me.