I always leave feedback, here on the Arena, on eBay and on Etsy (these are only sites I use that have a feedback system), so if I haven't left you any, please do remind me! I think Etsy's new feedback system is really bad now that you can only leave feedback after 200 years... If I get my item in the mail and I can only leave feedback three weeks later, I tend to forget about it. I don't like that.
To be honest I can't imagine how you can't have time to leave feedback, but then again, I don't buy, sell and trade nearly as much as many other people do.
Personally I get a little disappointed when people don't leave me feedback, especially because I don't have a lot. I have only 35 here, and on eBay I only have 11 (both since 2008).