Experience Training is when one trainer pays another to gain experience on their Pokemon for them.
These are usually advertised through "rates". - These rates are often 1:6 to 1:7 (meaning every 1 million experience trained, you pay/receive 7 million PokeDollars.)
This transaction is based on trust and credibility, so there is no strict system in place for releasing of Pokemon, etc.
Selling Pretrained experience:
These rates are when buying experience already trained.
1:4 is the standard system, where every 1 million pre-trained, you pay 4 million.