If you're already familiar with the trading in CSGO, the fundamental core of trading is not much difference to Dota 2. The platforms are similar, the methodology remains ultimately the same.
The only difference then lies with the items and evaluations of it.
Oh, as for keys, Valve has since steered clear from using generic "brown" keys as we call it from Dota 2. This was done in line with the introduction of Perfect World server, the server that is maintained by PW for Dota 2 users in China. The main thing about the brown keys, similarly to your keys in CS:GO, is that it involved a huge variation of luck, and thus, remedied as gambling by the authorities in PW. As such, it is in violation of conduct to actually use keys to unboxed as it constitutes as gambling.
The so called brown keys are now removed, and what we have, are specific chests that is directly purchased without the need for a "key". Each treasure chest will usually contain hero sets, and in some instances, a very rare drop. Each unbox yields you a non duplicate item set until the complete set is unboxed, in which the items in the chest are reset.
As for trading itself, we do not have specific sites like the one you mentioned. There exist so many items in Dota 2, that it makes it perpetually hard to keep track of it. This site has tried it's utmost best to keep track of all the items in the schema, but again, generality instead of providing it in terms of popularity.
As such, you can see the over saturation of the current market in terms of item due to ease of access, and also the lack of reward for collectors. There are a plethora of other reasoning's within the mechanics of item uniqueness that has been trumped by the community itself, as such, it is commonly termed that the glory days of dota 2 trading has well and truly passed.
However, there are a few unique items with a certain value attached to it that may pique your collecting interest.
As such, your enquiry as it stands seemed extremely ambiguous. Perhaps you can provide a little more specificity, else, the playing field is a little wide to just explain as a whole.
On a personal note, if you enjoy the game, you should purchase/trade for the item for the purpose of aesthetics, and worry less on ROI. The market saturation makes it extremely volatile to do anything of that at this stage and best to leave it until you have good grasp of the amount of meddling Valve has on Dota 2 economy.
NB: Also, steam market, your best friend for now.