The key is to have a relationship with folks at the store and to demonstrate that your fish are good quality (not too small, they are what you say they are, no deformities or disease, etc.).
When you're in the store see if they have in stock what you want to trade. If they're out, ask them if you could bring some to re-stock. Ask how many they'd like.
This is far preferable than showing up with a bucket of hundreds of fry.
I usually get 1/3 selling price credit from LFS.
To some LFS giving local people credit is preferable to paying cash + shipping to a wholesaler or farm. LFS attitudes toward hobbyists like me is a big determinant where I bring my business.