You would need a VERY large bio load to have any sort of large amount of cultivated bio media quantity to sell. Your best bet would just be to build a system without a pond to which just recirculates the water wthin itself with ammonia being dripped into the system at a specific rate to cultivate the max amount of bio media.
When people say, "My tank has cycled". That means the toxins in the water have gone from Ammonia to Nitrite to Nitrate. You can't say bio media is cycled because there is nothing to cycle. You can say its cultivated with bacteria though. Doesn't matter if some has more or less bacteria, if it has any, its cultivated, ready for use, ready to cut down on total cycle times if any.
The problem with this as well is the cost of the bio media to out weigh the cost of operation, maintenance such as water changes to keep Nitrate from going off the charts. Maybe setup a Nitrate filter of some sort instead.
Its not that the "media" wont be cycled. Media can not get cycled. Its your aquarium that gets cycled. Any amount of bacteria on bio media will quickly cultivate more bacteria in much less time than starting from nothing. You can see what bio media would be cultivated by looking at it. The bacteria is kinda tan or brown in color.
The main issue is that you do not know what bio load they have in their tank and what the bio load's needs are. So you would want to sell bio media which is has cultivated as much bacteria as possible.