Well,
Ive recently dealt with both of these methods,and 
each has their advantages.  For one, Id make sure 
you only accept serious offers.  If you have a 
cel you paid 400$ for, I wouldnt even write 
people back who dont at least offer 300$. I think 
one time offers are a good way to avoid hassle, 
but then again if one person makes a quick sale 
and all of a sudden can offer more, it would 
really suck that that person's already made an 
offer and cant up it. And...if you do take 
multiple offers....Id make a very strict amt they 
can up it by.  Nothing sucks more than losing a 
cel during an offer period by like, 5$.  
Personally I like the way Anichan does her silent 
auctions.  People email her with a price, she 
posts it, then others can try and beat it if they 
want to.  Its just like Ebay.  It is a personal 
preference though.  Both have their advantages 
and disadvantages.