I am in bigggg trouble. I have a beautiful office done in finished pine hung diagonally. Over the years we've hung diplomas, family pics etc. After 40 years in this home of ours - it's time to sell. When we took all the pics down - everything around them had patinaed and now we have all these light squares showing. I've tried sanding these areas with 220 grit and using Minwax Polyshades to darken them. I've mixed stains - no matter what I do, even if the light squares darken, there is still the edges of each pic space that don't blend. It's a big office and I was so hoping I could avoid sanding everything down and starting over. Even then, after all that effort - I still think the edges of each picture space will show. The office was originally done in Puritan Pine and urethaned in a Satin finish. After 16 yrs. The wood looks more like a dark golden oak. ( I tried that color. Didn't match.)
I sanded too much in one spot trying different colors and now that spot just sucks up the color getting much darker than all the rest of the wood. Too much sanding. We built this home and did a lot of the work ourselves. I'm no stranger to paint and stain. But, this is beyond my abilities. It's not like I can rip a board off the wall and experiment. Can someone advise me what to do?

Either hit it all with a belt sander, or paint it, or let it be and let the buyers worry about it! Put a few pictures up to give them an idea of what it looked like.
BUT
BIG BUT
The truth is that far, far fewer people want natural pine walls anymore, diagonal or otherwise. So I would take it all down...
If you don't believe me about what buyers want, ask your agent.

Nothing to do except strip it back or paint as JP says. Stripping poly is a pain. I suggest paint as the cheapest option.

Another vote for simply painting it. But I agree with Joypulv, I would ask my agent first and I'd do whatever they recommended for the best sell.