Database Design - Custom Attributes / Columns
A small JSON blob on Items to cover the wide range of descriptors of a item might be prudent. The rest of Orders / Clients tables should be fairly structured and not need any attributes.

Considering attribute tables as a primary design will get you into a mess when queries 3. EAV Tables — Entity-Attribute-Value Tables. A sharded Attribute table between a variety of other tables will make this even worse.