I've dealt with him as well, but not quite as 
recently as you have.  So I have a couple 
questions.

When I dealt with him a couple months ago, his 
exchange rate was 110yen/$1, and he charged $20 
flat for EMS shipping, regardless of the number 
of cels ordered.  He also required that payment 
be sent via Global Priority Mail.

Were those conditions the same for you?  Because 
if they were, going through celga may be a less-
expensive option for the private auction, if the 
cels cost less than 10000Y.

**The following information *assumes* that Celga 
would charge you $15 for EMS shipping, (source of 
assumption: 
http://www.celga.com/current/fee.html), so take 
the following with a grain of salt, and contact 
Celga directly for shipping rates.

Let's say, for example, that you won a cel in the 
private auction for 5000Y.

If you dealt directly with the seller at his 
exchange rate, you'd pay:

$ 45.46 for the cel (110y/$1)
$ 20.00 for EMS shipping
$  3.00 for an IPMO
$  5.00 for global priority envelope
--------
$ 73.46

If you bought via celga and paid with PayPal:

$ 43.10 for the cel (116y/$1)
$ 10.00 celga commission
$ 15.00 EMS shipping from celga
--------
$ 68.10

The prices are so similar (give or take a few 
bucks), that it hardly seems to matter whether 
you order through celga or directly through the 
seller.

Also, before celga's auction site, the seller 
sent me one catalog free-of-charge, and offered 
further catalogs for a small fee (I think it was 
$10 for two months, but I really don't 
remember).  So is celga offering a good service?  
Definitely yes, in my opinion.