What does Small, Medium and Large mean?  Am I a 4, 6, 8 or size 10? 

When buying or selling clothing on eBay you cannot assume that because you (or your buyer) wear a Small, for example, in Sigrid Olsen, that a Small in Calvin Klein or Eileen Fisher will fit the same.  I view the tagged sizes as a guideline, rather than a given. 

It is always best to have measurements available.  

Tops &  Sweaters

• Bust measurement should be from the seam under one armpit to the seam under the other.  The item should be laying flat when measured, or hanging.  The key is to not stretch the the article. 
• Length should be from either the back neckline to the bottom of the hem, or from the shoulder seam to the bottom.  The description should tell you where it is being measured.  This is not a commonly asked question, so you may need to inquire with the seller if need be.  I personally ask this to determine if I need a Petite or a regular size.
• Sleeve length is from the shoulder seam to the end of the cuff.

Pants / Slacks / Jeans

• Waist should be with the pant closed (if not elastic).  It should be measured with the full aournd-the-waist amount.  As an example, I fold the pants at the sides, like you would do when holding them up to you in the front.  Take the measurement from side to side, and double it.  Be sure you only measure the inside area of the band -- your tape measure can be on the outside of the pants, but you will see where the "open" area begins.  If you include the waistband thickness you could distort your measurement.  Make sure the measurement is without any stretching.   Also included may be the total stretched length, especially on an all estastic waistband.
• Rise is measured from the top of the waistband to the crotch seam.  This also helps to determine if something is low-rise or mid-rise.
• Inseam is measured from the crotch seam to the bottom of the hem. 
• Hips are probably the hardest item for a seller to measure, because hips can vary so much from person to person.  I try to give hip measurements from the bottome of the zipper &/or the bottom of the side pockets.  If either, or both, of these are used it should be meantioned how far down from the waistband this measurement is.

Skirts

• Waist and Hips should be measured the same as above.
• Length should be from the top of the waistband to the bottom of the hem.