I'm adding Schema.org (using Microdata) to a product page. My client wants the price omitted for sold products - she doesn't want anyone to be able to see the price of sold products.
However this was showing up as an error in the Microdata for 'price': check page in Google SDTT
Obviously the average user will still not be able to see the price, but it's used by Google Shopping so might show up.
Should I just put a price of 0?
It’s perfectly fine to have an Offer without a price.
If Google’s Structured Data Testing Tool complains, it does not necessarily mean that you have an error in your Microdata or Schema.org. In your case Google’s error message just means:
If you don’t provide a price, we won’t be able to show our Products rich result for your product.
The point of this rich result is showing their users data about your product, including its price. If you don’t have a price, there is no point for Google to display it. Don’t provide a fake price (like 0), just omit the price property. Otherwise you convey that your product has a price (in case of 0, that the product is gratis), which would be misleading.
