I buy cards with either a short message or blank inside. I’d say I’m more likely to be put off buying a card if it has a message that isn’t a good fit for me than one that doesn’t have a message at all. I’m happy to buy ones with something short like ‘Happy Birthday!’ inside if it fits the occasion I’m buying for, but I’d be put off if the wording was something I wouldn’t say myself e.g. if it was quite saccharine or religious. There’s a risk of limiting your customer base with a message that isn’t universal.
For selling, I used to have text in some of my cards, but these days I sell almost all them blank inside. A large part of the reason for doing it that way is because quite a few of my designs would work for a range of situations, so I don’t want to restrict them to one particular occasion; e.g. if I write ‘Happy Birthday!’ inside then no-one will buy it as a thank you card. It’s tricky though; I’d be interested to know how many people avoid buying cards that are blank inside.