I would definitely keep everyone posted during 
the offer period; it's how I run my offer-
selling, and it's how I'd like everyone to run 
it.  This way, if you're making an offer on a cel 
whose price you're not sure about, you can at 
least get a sense of what you're up against in 
terms of competition.  And also, there's always 
the chance that you can scrape up a little more 
for your offer in case another, similar bid comes 
along.  If you decline to offer more, then at 
least you can feel that you chose to pass up that 
cel, rather than have it "stolen" out from 
underneath you.  I'd much prefer to feel like I 
chose not to buy something, rather than that I 
missed out on it.

I would keep people posted over e-mails, however, 
as it's something I feel should be kept private.