Your cabinet bottom doesn't look that bad. Are you trying to hide the fact that you had a prior leak? That is a pretty common event - and you will be required to disclose any prior water leaks in a seller's disclosure statement. A home inspector will be able to differentiate between a current water leak and evidence of a minor old leak.

If you want to repair it just so it looks better, there are MANY "how to" videos if you do a search. Most of the water damage issues look much worse than yours. Your damage looks like it could just be sanded and then primed + painted.

Prime with Kilz® or BIN 123® and paint over with a high gloss paint, just the bottom of the cabinet. It will brighten up the interior of the cabinet and seal out the old stain. Won't look like you are hiding anything.

I'd put down some pretty contact paper, but be sure to line the other vanity, too, so it doesn't seem like you're hiding something. Lots of people line with contact paper under sinks to prevent damage from small leaks.

You still need to disclose it on any seller's disclosure affidavit required in your jurisdiction - regardless of whether it's discovered or not by a home inspector. It won't cause a purchaser to decide to not purchase your home.

For the love of heaven. You had a leak and you fixed it.

Put down contact paper in both, and move on. Most folks sell a home while it is still inhabited, and if you believe any buyer, or home inspector shall ever be peeking under the cleaning supplies, or the shampoo stored in a sink cabinet, you're nuts: )

No one expects a home of any vintage to be "brand new" in every regard

In my own homes that I have bought if the damage has been more extensive I have cut a piece of plywood to fit the bottom of the cabinet, installed it and painted the inside of the cabinet with semi-gloss paint. The damage you have isn't that bad. A potential home buyer might ask about it but you can disclose there was a leak and it has been repaired.

I would do nothing. I would be willing to be most vanities in America have some base damage from water leaks, spillage, something. I would be more concerned if it looked like a fresh coverup and wonder what else you may be trying to hide.

This is not a deal breaker to any buyer! Plenty of sinks have that stain as pipes leak. That is where one would expect to find a mark. The job of their inspector will be to see if that pipe is now secure. You could do any or all of the above, you could also just fill it with cleansers, toilet paper, WHAT EVER....no one is going to look. I do not think this is a disclosable event. It's common and did not do extensive damage and is not likely going to happen periodically the way a flooded basement might be.

I would clean it, sand it and prime it and give it a good coat of paint. You can also buy peel and stick floor tiles and put them down nicely under there.

Nobody is going to look unless you are moving before listing!!

You ”disclose ” nothing. The stain is common as dirt, unless you protect when first installed.

Jeeeesh!

Place clean contact paper and a nice trash can down there and be done with it. It is dry and the leak repaired. I want all of my cabinets to look clean and tidy beneath when I list a place.