If you get this error message:
The Legacy XML format used in this request is being phased out. All applications should migrate to the Unified Schema to avoid loss of service.
Please ignore it.
This is the technical explanation:
The error is refering to a change eBay made to their API program. Legacy and Uniform Schema to be specific. This 'error', does not effect your listing... but it is eBay's way of telling third parties that we need to migrate to the 'new' Uniform Schema. We are almost completely migrated, and the deadline is June 1, so everything is fine.
The non-technical explanation:
Ignore this error... in fact we probably are just going to mask it (you won't even notice that eBay returns this error).