I understand that you are still learning about "linoleum", so I'll bring you up to speed on the subject.
Basically, there are really two kinds of "linoleum"; real linoleum and sheet vinyl.
And, there are two kinds of sheet vinyl; paper backed and vinyl backed.
Real linoleum and both kinds of sheet vinyl will become stained if you spill the wrong thing or put the wrong thing on them, but the higher end sheet vinyls that have a polyurethane wear layer are more resistant to staining, especially from sponge backed bathroom throw mats.
Real linoleum is by far the greenest product. It is made by mixing linseed oil with chaulk, ground up cork and coloured particles called "pigments", which, except for the pigments, are all natural materials. The resulting flooring hardens with age just the same way that oil based paints theoretically continue curing (and getting harder) forever. Marmoleum is nothing more than real linoleum with an acrylic floor finish applied to it at the factory.
Sheet vinyl flooring comes with either a paper backing or a vinyl backing. In both cases, they are "full spread" glued down to the floor, which means that the glue is spread over the whole floor under the sheet vinyl, not just around the perimeter of the floor.
My personal opinion is this: Either the day your flooring is installed, or the day after the warranty expires on your sheet vinyl or real linoleum floor, you should apply an acrylic sealer to it to protect it from stains. Read your guarantee. Often the guarantee does more to protect the manufacturer than it does to protect the customer. That's because the guarantee will establish the limit of the manufacturer's liability in the event of a defective product. That is, often the guarantee will state that the manufacturer is only responsible to provide an equivalent amount of new sheet vinyl or linoleum to you free of charge if the stuff you bought proves to be defective. It's up to you to take the old defective stuff out and put the new stuff in, so you're paying most of the cost anyhow. (And, that's probably still true even if you install the new flooring over the old defective stuff.) So, if being given replacement flooring free isn't a big issue with you, I'd apply an acrylic sealer to your new flooring ASAP. Clean the floor as best you can before sealing to avoid burying dirt under your clear acrylic sealer.
If it says that the manufacturer will replace the flooring at no cost to you if it has any defects, then wait for the guarantee to expire, and then seal it. I own a 21 unit apartment block and I have sheet vinyl flooring in all of my bathrooms, and the only stains on those floors are from the time before I started applying acrylic sealer to them to protect them from stains.
I like a product from Buckeye International called "First Down". This is an acrylic sealer meant for vinyl composition tiles, but works equally well on sheet vinyl flooring. I would not recommend putting it on over Marmoleum because it dries to a harder film than the acrylic floor finish that's factory applied to Marmoleum. If it were me, I would simply apply MORE acrylic floor finish to Marmoleum to protect it from stains.
And, all acrylic floor finishes and sealers are no more environmentally harmful than latex paints, acrylic grout sealers or acrylic nail polishes. That's because they all form films through a process called "coalescence", which requires the use of a coalescing solvent, but in a much lesser quantity than the thinners used in oil based paints. You will do no more harm to the environment by applying an acrylic sealer to your floor than you would painting it with a latex paint.
The way I seal my sheet vinyl floors is with a "lamb skin" or "lamb's wool applicator", which are normally used for applying polyurethane finish to hardwood floors, and which you can buy at any place listed under "Janitorial Equipment & Supplies" in your yellow pages phone directory. Lamb's wool applicators used to be made of real lamb's wool (and you can still buy them made of wool) but nowadays most are made from rayon fiber because it works equally well and costs much less. Lamb's wool applicators come in various sizes and are installed in a holder to apply polyurethane to hardwood and gymnasium floors like this:
However, if you intend to use it with a water based product like an acrylic sealer, then you should soak it in water first, squeeze out the excess water, put it in the holder and only then use it to mop on the acrylic
sealer. If you don't wet it like that first, then air will be trapped in the fibers and will cause the acrylic sealer to foam up when you're applying it. Keep the applicator sealed up in a plastic bag between coats to prevent it from drying out and hardening up. Removing any printing from the bag with nail polish remover (acetone) first as the coalescing solvent in the acrylic sealer can dissolve this ink, causing it to add unwanted colour to the sealer.