I've not sold or traded cels either. I'm just a 
buyer/collector. I've been in for 3 to 4 years. 
I've bought from lots of internet dealers. My 
response to your questions are as follows:

1) From receipt of payment, one week maximum, 
which is surely enough time for seller to realize 
he/she will be delayed. I have never encountered 
this problem, though.

2)This is a good one. It has happened to me, but 
on reflection I realized that the cel price was 
cheap for this dealer and if he were to offer to 
take the cel back, I'd rather keep it since I am 
not in the business of reselling and I probably 
won't get another chance at a similar cel at that 
price. However, if the defect bothers you that 
you don't want to keep the cel, then you should 
notify the dealer. Keep in mind that cels are 
used goods, not designed to last after use, and 
typically passes many hands before they reach 
you. 

3) Makes no difference to me