eBay has stated that it is okay for the seller to request feedback from the buyer before returning the favor.  (I am not quoting eBay -- just paraphrasing.)

When I began selling on eBay, I would leave feedback immediately after receiving the buyers payment.  About half of my buyers would reciprocate.  I have found that there are many members, after reviewing their "feedback left", that did not leave feedback for anybody.

I resorted to requesting feedback first.  "Please remember to leave feedback, after which, I will be more than happy to do the same.  Thank you."  That is the statement I included with my packing slips.  Still, I had only about half of my buyers responding.

The following is the statement which I now include on my packing slips,:  "Please contact me when your purchase arrives, so that I know that the transaction is complete and I can leave feedback.  Have a great day!"  I have had a great responce with this statement.  About 85% of my buyers are more than happy to contact me and leave feedback.