If you're buying clothes on ebay and unsure of whether the sizes will be correct, ask what shop the clothes originally came from. That way, you can go and try on some items in that shop to get your correct size then you will know what to bid on!
With trousers: Most sellers give the waist size of the trouser, but beware of buying before you check the LEG length, I have bought trousers which are too short for me and have ended up having to resell, so email the seller and check!
With shoes, particularly sandals and flipflops: I fluctuate between sizes for flip flops so the easiest thing to do if ask the seller, if they wouldn't mind, measuring the sole of the sandal and giving you the size in inches, that way you'll have a clearer idea of whether it will fit!!