Specialty good.
In analyzing consumer patterns and consumer goods, typically goods are divided into three main categories. Convenience goods, shopping goods, and specialty goods.
Convenience goods are the sort of things regularly sought and used by consumers, and thus conveniently available from many vendors. They can be daily necessities (eggs, milk, etc), or non-necessary items that nevertheless are frequently sought by consumers (snack foods, soft drinks, etc).
Shopping goods are the sort of things people shop for in a more planned way, making comparisons, finding the best product to suit their needs, etc. Examples would be clothing, furniture, appliances, etc.
Specialty goods (such as a specific painting for an art collection, in your example), are unusual or unique or luxury items that are items of interest to a special group of consumers.
