Once the transaction is complete, the buyer is not obliged to come back - what are some good tactics to get the seller to come back to rate the transaction?
If the buyer is getting a discount or is signing up for a Beta program, you can make the feedback as part of buyer's contract.
If the buyer is purchasing a regular GA product, providing additional discounts, gift or coupons for their feedback is another method. Something to acknowledge the time they are taking, is often successful.
Depending on which marketplace you're talking about, but I know that Amazon doesn't want you to solicit any of their buyers since they already do it for you. On eBay, you can usually ask for feedback.
A tactic I've used is to include a reminder note within the package or packing slip that reminds the buyer how important feedback is for you and the marketplace.
I don't recommending using any software to request feedback because it's looked down upon by marketplaces is general.
Hope this helps