I do not count my self as a specialist on DC , so please take it as my personal opinion based on some experience in tea biz.
From what I follow on not only foreign market, the price doesn't really have to reflect the quality or grade ( of course nobody sells the high quality for cheap, but some do other-way around ) . You never know the vendor's expenses. You don't know who buys from farmer / producers ( on internet can pretend anything ) for what price ( it could be all different no matter they claim about some direct sourcing etc. ) Also you don't know how much profit they require, also different with any vendor.
I see many threads from tea drinkers "worrying" to be ripped off or not sure about the quality of teas they have already purchased , searching some answers on internet forgetting the one important thing. The price and quality judge is only You
If you like DC teas , I'd suggest to spend more time on google than asking on forums where same vendors are recommended over and over ( usually by same people ) .
I can speak only behalf of my self and that's
what I would do if search for optimal vendor.
Choose 1 or few names ( in particular type of tea ) , so for example as mentioned here Duck Shit. Find the other names used for that , like CN one is Ya Shi Xiang , and google as many as sites possible with keywords suitable for your purchase ( means if u want shipping in you country or don't mind to get it from China , or even directly from the place of production ) .
I would pick few vendors and buy samples of exactly the same model. Please note that DC also have various types of processing , dark, medium light and if older also storage is imprinted into the taste ( like dry or wet ) . If you are passionate about it you can learn some CN terms used like Xue Pian Wu Ye ( something like Snow Flakes Dark Leafs ) , which basically stands for winter harvest and supposed to be the best one for Duck Shit teas ( like spring for green tea ). Winter harvest is also quite often processed with light roasting coz leafs are rich in taste and no need to enhance it with nutty ( heavy roasting ) notes.
I would compare it like early spring "tou chun" harvest and puerh processing. Nobody is using 1st flush making a shu puerh ( economically also not effective ) , since it's already good as it is in sheng version.
Chi Ye or Bai Ye are best in autumn..etc. There is a lot of CN terminology which might help you to orientate your self in DC world.
I would pick the DC which I didn't like at first place and try to find it from different vendors , compare, maybe find out that the 1st one was low quality or brew it wrong way.
Judging the quality is about the practice , finding the right vendor is about the try & fail .
There is also lots of commercial hype around the DC and marketing strategy from vendors ( like free shipping or XX% discounts ) don't make the see t
he real price easier either. You can check how much is the shipping from same vendor by looking at their items without "free shipping" ( please mind that pressed tea and fragile loose leaf require different packaging , so look for similar character of the product , and weight of course ) .
I would also spend some time with narrowed selection of vendors and read their "About us" or their blogs, to get a picture who are they.
Some vendors just re-sell tea as "shoes" , like some thing...no personal attachments, if u know what I mean. Some vendors go with Traditional CN tea culture and all that commercial "beauty " around the tea ( not my favorite one ). I think it's good for tourists, not for actual dedicated tea drinker. Some vendors are big , with directors, managers ,CEO etc., some of them just simple family shop or factory. Again, please note, internet is very confusing in those aspects, I see many vendors on TB hugging a fat tea tree claiming to be a farmer / owner of this and selling sheng pu-erh directly
))
So to sum it up, it all boils down to you very own buy & try concept
Have fun;-)