Below are instructions on how you buy a COC account on either IOS or Android devices.
Iphone/Ipad
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1) Link CoC account to GameCenter (seller)
2) Create a new GameCenter account (buyer)
-- When creating the GC account the seller provides the buyer with their AppleID.
-- Once GC is setup it will it asks the buyer to link to the CoC account
Android
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1) Link CoC to Google+ account (preferably a new Google+ account the buyer creates)
2) Log into the same Google+ account on your device and restore CoC
If the CoC account is already linked to someones personal Google+ account, then it becomes difficult because the original owner will always have access to the Google+ account.
I'm not familiar enough with the process to know if a CoC can be unlinked from a Google+ account. If it can then there should be no problems since the seller can always create a new Google+ account and just give you the G+ account to secure it.