Can you correct me?

買いたかった本が届けなかったから、ちょっとがっかりしましたが、結局、他の売り手で買いました。

To say: "Couldn't be delivered" would you use:
intransitive 届く in potential form 届けません? or the transitive 届ける and passive form 届けられません + potential? (like 届けられることができません)

Anyway, maybe it's totally incorrect.

Sehen Sie eine Übersetzung

First of all, in Japanese, a verb can be changed into a potential form only if the subject to the respective verb can willingly act.
(Have you ever heard volitional and non-volitional verbs? https://japanese.stackexchange.com/questions/82... )
Given the sentense you write, the subject to 届く is 本 and since 本 doesn't willingly initiate the action, it is impossible to make the verb potential way. In such case, we usually don't consider we should use a potential form or not, reather we try to state the fact that "the book wasn't delivered" (I'm not sure if this English makes sense here) 本が届けられなかったから. (Be aware that this られ is passive form, not potential form)　
Or we most likely say 本が届かなかったから. This means "Since the book didn't come" (again, I'm not sure if this English makes sense here)

買いたかった本が届かなかったから、ちょっとがっかりしました

I hope my explanation makes sanse. Please let me know if you need me to explain more.