Even if you personally aren't fussed about spelling and grammar, your online store can lose credibility and therefore business if it contains spelling and/or grammar mistakes. One single spelling mistake can cut online sales in half!
I will usually leave a website almost immediately if I notice any obvious errors, and I'm sure I'm not alone! My thought process is that if they can't get their website right, how professional is the business itself? This is especially true if it's a professional type business.
You have around six seconds to capture your website audience, so make sure it's error free!