A seller providing the weight is very useful when you’re familiar with the specific brand/model, and you know the range of weights they come in, and you have a weight in mind. E.g. there are a ton of Paiste DE 22’s in the 3100 g range, and maybe I’m more interested in buying if it’s under 2900. Or maybe I’m making a set of 50’s A hats in a New Beat weight ratio, and I already have one of them, need to find the other in a certain weight range. Or I’m into pre-split Istanbuls and I see a 20” ride but it weighs 2600 g, or an 18” crash but it weighs 2000 g, I’ll know not to take a gamble on those.In over 45 years playing drums, I've never bought a cymbal or searched for a cymbal by their weight. Unless you're planning to recycle them, but you'll lose a lot of money selling them for the metal.
I guess if there's no markings anywhere on the cymbal, the weight might be helpful, but the bell, taper, and thickness would be more helpful determining the type of cymbal.