First and foremost, invest (a couple of dollars at the most) in a measuring tape! Buy the cloth kind (rather than the metal). These are available at any fabric or craft store, as well as any drug store (Longs, Walgreens, etc.) and even most grocery stores.

Second, always provide measurements of any garment you are selling in your item description. Why? Because simply saying "Size 20" or "2X" (for example) tells the buyer very little about the actual fit of the item.

One manufacturers "Size 20" or "2X"  may be another manufacturers "Size 32" or "4X", etc. The size tag often has little to do with whether or not the garment will fit the prospective buyer.

I can only speak for myself, but depending on the actual measurments, I can wear anything from a size medium to a size 32W.

I have passed many items by on Ebay because the description was vague or incomplete.

HOW TO MEASURE

For width on tops (and dresses), measure across the bust (underarm to underarm); across the waist area; and if it's a longer top (or a dress), across the hips. If it's a stretchy fabric, make sure to say so. Mention any vents (side, back) it has and how long they are. If you flat measure across the front of the garment, and it comes out to be 22" (for example), make sure you state that it would be 44" around.

For length on tops and dresses, measure down from the shoulder seam to the hem. Measure the sleeve length from the shoulder seam down the outside of the sleeve to the edge of the hem or cuff.

For width on skirts, measure across at the waist. If it's an elastic or otherwise expandable waist, measure both relaxed & stretched. Measure width at hips.

For length on skirts, measure down from the top of the waist to the hem.

For width on pants or shorts, measure across at the waist. If it's an elastic or otherwise expandable waist, measure both relaxed & stetched. Measure width at hips,

For length on pants or shorts, measure from the top of the waist to the hem on the outside leg seam. Measure the inseam from crotch seam to hem.

This will save you a lot of time in answering questions from buyers who will want to know the actual measurements of the garment you are selling.

Always provide the fabric content of the garment (100% cotton; 50% cotton, 50% polyester; for example) and the cleaning method recommended by the manufacturer (machine wash; dry clean; etc.)

Always describe whether or not your garment is new. If it is not, describe any flaws it might have, no matter how slight.

EXAMPLE OF A CLEAR AND CONCISE ITEM DESCRIPTION:

Item: Forest Green Short-sleeved Velvet A-Line Dress. Sweetheart neckline.

Size: 24W (Bust: 50" around; Waist: 55" around; Hips: 60" around; Length: 50"; Sleeve length: 12"). 16" back zipper.

Fabric Content: 50% cotton, 50% rayon

Care Instructions: Dry Clean Only

Condition: Like new, worn only once to a party. No stains, rips, or other flaws. Non-smoking home, but we do have cats.

The above tells the buyer everything they need to know about the item.