That is perfectly fine. A test server is not counted as an end product. You can use it in your development process as many times as you want it.
It’s very kind of you to respect the licensing, very few people value the work put in these stock products.
On a related note. I’m not sure if you are familiar with the referral program envato offers, which would benefit you I believe. Here are some details: http://themeforest.net/make_money/affiliate_program
Basically when you tell your customer to buy my or any other authors plugin, you can put your name as the referral, and earn a comission of 30% of their initial deposits as envato kredit (which you can use to purchase items). I’m not sure if you need to become an author for that, but it’s worth a try.
For example the link to Ajax Search Plugin would look like this with your username: http://codecanyon.net/item/ajax-search-pro-for-wordpress-live-search-plugin/3357410?ref=enoversum
So basically you add the ?ref=enoversum to the end of any codecanyon or themeforest page and the newly registered user becomes your referral.Best,
If you like my products, don't forget to rate them on codecanyon :)