I have been told that instead of staining and then putting poly on that I should get the poly with the stain in it. 

The walls have stains in different places and I can't clean them due to the flat paint they used. 

Not a nail hole or carpet stain to be found.

It will make a great investment for someone willing to invest the money - just not a fit for us right now.Takeaways:For our first house hack, we've got two ways to go: 1) purchase turn-key. or 2) purchase a fixer-upper with strong fundamentals (i.e. the carpets are stained, and the place stinks, but the roof is in good shape).

I have tried everything to get these stains out and no luck.

@Dave Charron I either Paint white because I like the farmhouse look it gives or I just do a nice stain on them to make the wood pop out more. 

Sand, stain and poly the existing wood floors (and obviously have something written into lease regarding protecting them within reason - keeping temperature and moisture regulated.)2. 

soap for fats and sugar, I use Coleman stove gas (white gas) for tar or oil based stains, and water for most anything else, go easy on the water as you don't want it to get on the other side of the polyurethane (through cracks and grooves). 

(Paint, floors sanded and stained, carpet in bedrooms.)1.

If it is just top layer some sand off the sub floor stain and reseal.