It is often difficult to keep consistent results when exporting rigging information to other formats.
Exporting a rigged model to an exchange format might result in some information loss (i.e. constraints, controls). To avoid that you may want to prepare a model having just the basic rigging. To keep the most essential information, correct import/export settings must be set.
If the rig doesn’t work when using an exchange format, the seller should state that in the product description.