A few months ago I noted this eBayer (ron_ball) was selling stamps that were not what they were advertised. I provided him with a link to my NWPI overprint thread here on Stampboards where he could post and vet his items re: their genuine nature.
He did promptly reply with a legitimate reason but has not posted any of them on my thread for comments.
Usually the eBay listing is a combination of often used fake & genuine stamps. If the former the significant glaring characteristic of them is the Australian postmark.
Although not definitive since the overprinted stamps were: 1.) sometimes used in Australia & 2.) they might have been accidentally cancelled while the letter was in transit any non-New Guinea cancel should be taken with suspicion unless one is well informed of the postal history of the area.
Additionally any overprint where the 'P' of 'PACIFIC' is offset too far right to be:
1.) directly above the first 'S' of "ISLANDS' (SG 65 - 101)
2.) above the centre of the 'IS' (SG 101 - 124 & O3 - O21)
approach with caution.
Here are the current items on offer (as of time of posting). Before I comment on the fakes let's see if anyone can spot them.