An eBay transaction isn't really complete until the buyer indicates that they have received it safely in good working order, so from that point of view, I would side towards the notion that buyers should leave feedback first.

Donna and kopenhagen are correct that the entire process is optional though, and digit*ghost is also correct that some buyers out there expect the seller to leave feedback first. In some cases, buyers can get quite irate if they are not instantly left feedback after paying.

To get around this issue, I state on all my listings that "positive feedback will be left once positive feedback is received". It might be worth including a similar disclaimer on your listings if you are worried about buyers demanding that you leave feedback first.