As far as measurements, it depends on the style of dress, but at the VERY least: Bust, waist, hips, shoulder to waist, and skirt length. If the dress is a very stretchy knit or lycra, add the unstretched measurements and a "reasonable" stretch. Also note if the skirt has been hemmed, and if not, how much selvedge edge is there just in case someone loves it but its too short for their height. If its a halter, measure the width of the front panel and let the gal decide how much coverage she wants.
Nice to have:
If the dress is has an empire waist, or is otherwise fitted right under the bust, I look for the measurements of the underbust as well as the bust to see if it would be right for me. The measurement across the shoulders is a good one especially on more structured dresses. For me, this is not always a nice to have but a must have on certain necklines being petite.
Not crucial, but really Awesome to have:
If the back is plunging lower than where a reasonable strapless bra would be seen, measure shoulder to end of the plunge. I look for this to know if I can wear a strapless bra with it or would have to invest in a backless basque (and I decide how much I really like the dress based on how much more on foundations I need to invest.) Also, if the sleeves are very embellished and you would cut into an intricate design/wouldn't be practical to shorten, list the sleeve length from shoulder to cuff and underarm to cuff. Also above and beyond is if a skirt has a few inches of the selvedge at the waist band, I know if it can be let out.
Things that annoy me:
Unless the item has defined molded cups like a bikini, I get annoyed to see darted items sold where the seller expresses the bust size as a bra size, like so "Maybe would fit a 34 plus a B cup" I want to measurement to be the measurement with saying "this is the measurement, and the darts may add a few inches in the bust" rather than judging what boobage its for. I know what measurements fit me, rather than a seller not telling us the full inches.
A seller listing a vintage item with a modern number size in the title. S/M/L is fine, but when one starts trying to equate to a modern size, it never works because the proportions and fit is so different. It makes it really hard to find what I am looking for.
