Part of it is going to depend on the kind of cel 
you're buying... the way some deps calculate 
their commissions, it might be cheaper to use one 
dep for a high-end pricey cel, and it might be 
cheaper to use another dep for a mid-low-range 
cel.

Likewise, some dep services will require you to 
make a deposit on the item you're bidding on, as 
a way to protect themselves somewhat against 
flaky bidders. Some dep services won't let two 
customers bid on the same cel, so if someone else 
has already commissioned the dep service for a 
bid on the item you're looking at, you may have 
to find an alternate method. No two services are 
alike. It's all a matter of what works best for 
you individually.

Once you've found someone whose method you like, 
you might query about them specifically. You'll 
probably hear good stories and bad stories about 
any dep... I've had deps who have completely 
spaced bidding on my item for me for one item, 
but who have been perfectly helpful, friendly, 
and reliable for other items. ^_^ No dep service 
is going to be perfect... but it's a good way to 
get a feeling for the general attitude towards 
their practices.