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For online auction sites:
- Seller is reliable (i.e no accounts with zero feedback, no accounts that are only only a few days old, only feedback from recently created accounts)
- If the seller is on the border between suspect and legit or the price is too good, then make sure that the seller actually has the machine (meet him/her in person and if this isn't possible ask for a picture of the machine with the day's paper)
- That the specs of the machine match up with the machine in the picture
- That the battery is ok

Personally I wouldn't buy something as expensive as an rMBP without meeting the seller in person and then I'd check that the machine boots, doesn't have any clearly evident issues (like a ruined battery), doesn't have any outward damage and that I can have the receipt.
 
And always check the refurbished prices on the Apple store. You may find out you aren't getting as good a deal as you think.
 
Just a few things that readily come to mind...

1) When inspecting a used MBP in person, make sure that the "About This Mac" is accurate. There have been several cases of online sellers changing the stats of the aforementioned, in order to defraud buyers.

2) Check the serial number online for warranty status etc. You want to make sure it all matches.

3) Make sure FileVault 2 has not been enabled and other iCloud services are logged out etc.

* Ideally, the OS should be a clean install waiting for it to be registered with the new user and iCloud account.

4) Another big issue is underlying damage (from spills etc.) that can't readily be seen without taking the bottom plate off. That is something you will need to contend with. It is a risk. Unfortunately, not everyone is going to be honest with you about what they are trying to sell.


In my opinion, it would be better to buy from the Apple refurb store, or off the forum here from an established member that has good standing.
 
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