Zeppelin Values

Lately I have been watching C13-C15 on ebay just to see what I would have to spend if I decide they would make a good anniversary present for my wife. I'd let her keep them in my album of course .

I've noticed most of them are not for bidding (Buy It Now), but I've also noticed that with a PSE certificate, the price is significantly higher. Other than original gum, what's to be certified? Is counterfeiting an issue with these stamps?

There are some forgeries but they are fairly easy to spot. A bigger issue is 'changed gum'; where someone has messed with the gum (regum, gum smoothing, etc.) or possibly perf issues.
Don

This is old -- dated to 1984 -- but still should provide some answers to your questions:

http://www.philatelicfoundation.org...Zeppelin.pdf

IMO, one reason why this set is higher with a cert is the cost of the cert itself. High fees will always boost significantly higher valued stamps ever greater simply because the fees themselves are based on a percentage...a cost ultimately passed onto the buyer.

Price might be more appealing if you look for a nice stamp(s) from a reputable dealer offering a good return policy and if you have any doubts, you can always get it graded yourself. Most reputable dealers will allow sufficient time to do that and will even offer extensions on any return period guarantee if necessary.

I bought my C15 from Gary Posner last year. His prices are reasonable, and he grades/describes his stamps reasonably. He usually has a good selection of Zepps. Check out his website here:

http://www.garyposnerinc.com/

Edit: the higher value stamps like the Zepps that are offered in auction on ebay, as opposed to BIN, tend to be specimens with faults or poor centering. If you can settle for Fine with minor faults, some bargains can be had this way. But the quality specimens are more likely to be offered BIN, or with very high starting bids if offered at auction. Not everybody knows what they are looking at with a venue like ebay, so the best stuff gets auctioned elsewhere, or offered with floors on ebay to make sure the seller gets a reasonable price.