I was a seller in the marketplace themeforest for 2 months, I sold my template a lot and took it out of the air in the third month, it's worth it to get ready for support. Many users shop and want full support from a small detail to something you did not add to your product.
The values you can see this link .
Precious Tips:
Full documentation, explain everything about the product;
Code quality, the easier it is, the easier it will be to maintain;
Advertise on social networks, your customers are on social networks;
Preferably for Wordpress templates (best sellers);
Have patience, many customers do not know anything.
I wish you good luck in your sales, I want to soon resell in the network. I only have time.