I don't usually sell clothes, so this guide is written from the buyers perspective. 

Buying clothing on eBay can be tricky and since many sellers do not take returns, it is important to get measurements and compare it to something you already own.  Sizing varies widely and there are so many ways a garment can be the "right size" yet not fit properly.  When you are interested in buying a garment, take a similar piece that fits properly from your own closet and take measurements.   

When purchasing a shirt, jacket or a top I check 6 measurements: 

• Shoulders (seam to seam)
• Bust/Chest (measurements taken just under the sleeves of a garment),
• Waist
• Length
• Sleeve length
• Sleeve opening (Sometimes it is good to ask the sleeve opening size from older garments because they were narrower) 

When purchasing a skirt, I check 3 measurement: 

• Waist
• Hips
• Length

When purchasing pants I check 5 measurements: 

• Waist
• Hips
• Thighs (a measurement taken across the thigh of the pant - not many sellers list this measurement, but it's uncomfortable to wear pants that are too tight around the thighs and useless if you can't get pants on because the thighs are too narrow, so don't be afraid to ask for this measurement)
• Rise (measurement from waist to crotch)
• Inseam

These measurements take a minute or two to take, but really increases the chance of finding a nice fitting piece.  If you check these measurements and the you and the seller have measured accurately, the likelihood of recieving a garment that fits is quite good!