I buy RP only related items if I need them, though most of the things my characters keep on them for RP purposes only are gifts they've received.
Example A: Quinasa wears a wedding ring, and she also keeps a ring Athian gave her in her belt pouch. She has a night angels blossom that never fades tucked into her belt or wrapped safely inside her bag, and she carries always a harp that is worn with age and use.
Example 2: Sicily is constantly having to buy pickaxes from NPCs, even though I RP that she has one trusty pickaxe that she adores and keeps maintained. Also if she sees something she likes but will never use and still wants to have just to have it, she will buy it if she has the money. She likes having things, even if they have no use but to be owned and looked at.
I don't have any characters that have a lot of money, so I don't have so much money that I require spending it. If any of my characters start to have an absurd amount of coins, I'll spend it on foods and tools, or start learning a new craft so that my character has an in game reason for spending the money.