[Resolved]

I'll copy paste some of what I messaged @Sisig on Discord that sums this all up.

No terms were agreed upon, so the seller (Cooper) can not use his ToS to force a different ruling than it would normally be done on Sythe.
For example, a warranty period stated in his ToS is invalid, instead you'll now have a warranty of 2 years, as that's the standard on Sythe when no ToS is agreed upon. However, anything he states on his thread, including his ToS, can now be seen as part of the advertisement. You could've read about the 50% buy-back thing, and it could've been a factor for you when you purchased the account.
This is in favor of you, the buyer.

Now on his thread he has stated the following: ''4. If you choose to resell, you may ask me and if I choose to rebuy, it will be for 50% of original sale price.''
This states that you may ask him, and he will re-buy it if he chooses to do so. It's not him stating that he will always offer a buy-back. It leaves him room to change his mind. It only says that if he wants to offer a buy-back, he will for 50%.

Cooper has stated on this thread that he will buy it back from you when he has time to do so. When exactly this is, is up to him.