Finally decided to start buying stuff here (plan to buy the phb, xanatar and tabaxi from Volo), and I was wandering if there are ever any sales. If yes, where and when can I find them? (black Friday, I know ddb is often advertised during podcasts and stuff...) 

They pretty regularly release discount codes.  There will almost certainly be one for black friday/cyber monday although it has not officially been announced to my knowledge. 

There is a forum thread here that is pretty good about keeping up to date about the discount codes.  I don't think there are any active ones right now though.  I'd give it a week or two.  One will likely pop up.

Another 2 questions if you don't mind.

1) Do the sales and promo codes stack

2) If I want to buy a race (Without the rest of the book), do I still have to purchase the subraces separately?

1. I don't think so.
2. You'll either have to buy the book or the subrace.  Only the items found in the basic rules (and maybe some playtest content) are free otherwise everything else costs money.

To clarify that second question just a little bit, you can buy a race (or subrace, magic item, monster, etc.) by itself without the book, or you can buy all races (or subraces, magic items, monsters, etc.) from a particular book without the book. Generally individual items are $1.99 while the collection of all of one type of items together is somewhere in the neighborhood of $5 - $15 depending on how many items are in the list.

For example, the Tabaxi race by itself is $1.99, all 12 of the races from Volo's without the book is $9.99 whereas the whole book is $29.99.

And also, if you buy things piecemeal and then decide later you want the whole book, the cost of what you already paid is counted towards the final price.

So if you buy just Tabaxi for $1.99 and then later want all of Volos, it will cost $28

I meant that I am confused by subraces costing the same as the whole race (i.e. the changeling subraces in https://www.dndbeyond.com/marketplace/source/eberron-rising-from-the-last-war). If I bought the changeling race, do I get only the basic race and no subraces? Or do I get all subraces and the single subrace is simply a bad deal?

Changeling does not have subraces. Shifter does (Beasthide, Longtooth, Swiftstride, Wildhunt) and so I think you would need to spend $4 to get a functional Shifter character. I have not bought any content individually though, so I can't say for sure.

Subraces are independent of races in regard to purchases. This may in part be due to some cases where subraces are from entirely different sources (for example, the Sea Elf in MToF).

Quote from Gvarph >>

Another 2 questions if you don't mind.

1) Do the sales and promo codes stack

2) If I want to buy a race (Without the rest of the book), do I still have to purchase the subraces separately?

1.  They did last year iirc.
2. Yes, you need buy the race and its subraces separately.

Any news regard Black Friday/Cyber Monday?  I might consider a couple more adventure books or may be even a package if things go on sale.

Another thing to keep in mind is the discount codes usually only apply to purchases of a book-level resource. Meaning, if all you want to buy is a race, don't expect a discount code to work. There might have been one or two that have (check the fine print) but 9 time out of 10, they haven't.

There is currently a sale going on right now.  No code required.

Any new sales?

Not right now. You will see a blue banner at the top of the screen with a discount code if there was one.