I don't worry about rust in the bed! Ain't got nuttin' to trap the water under, no spray in to develope a hole and allow water to seep underneath, no worries along those lines at all.
Scratches? I have oodles of them! Dents? There's a few! Worry? Nope! The inside of the bed may look like he.. , but that is NOT a worry...it just means the truck has been paying for itself.
I will sling something over the side of the bed and let it thump down, not caring if it 'mars' the finish vs oh so carefully spending extra time to place it 'gently' in the truck. Concrete pier pads, cinder block, whatever.
A few times, that 1/2 ton has had around 3000 lbs in it of various materials, and waddled off home. Works for me!
Many of you have seen the pics of the RAM and Ol' Grumpy, and I cannot stress the fact enough, it looks good on the outside, and doesn't hurt the value of the p/u. Plus, you save time and other costs, ie, you don't tear up your liner, or worry about the insert blowing out of the bed at 70mph going down the hiway. Seen it happen a few times...and they don't hold up to 18 wheelers worth a hoot.
P.S. - Save your bucks, and when you are ready to move on to a new truck, then spray a bedliner in at that time...if you still want to.