How would I reverse yellow stains resulting from sealing a marble table top?
Sanding even with 220 grit requires feathering and also making sure you go to 600 grit or higher to match existing polish. A product I use as a professional called diamond renew works wonders it is acidic and it is similar to glass etching products.

It must be applied and removed in sections exactly as the instructions say.

As warning: it is a lot of physical work and  it must be used across the entire surface after sanding. 
 It is a difficult process but renews the surface to a factory or near factory finish. You must not let it dry on the marble or get into cracks  as it will continue to eat in to the stone and create pits. 

If after feather sanding the spot another option besides diamond renew you might try to buff the surface with  rouge. 
Use a variable speed buffer on low to medium speed with enough rouge to bring out the shine but not so much it leaves residue on the surface. Start light and work up.

However diamond renew is my preference it is just very difficult and you should if possible practice on a piece of marble before you attack the counter top. 

Lastly marble is not granite...lemon juice can discolor marble and it scratches easily so sand lightly ....if you are not sure how to feather sand do not attempt it until you understand it. Also please remember products that claim are for both granite and marble should be used with caution. I would recommend using an "ager" after you finish sanding and polishing...not a sealant.