1. Unless you have a light-box, take your photos in daylight if possible. Daylight is always better than electric light or camera flash to get true colour. This may not be possible in the winter for daytime workers, in which case use the whitest light that you have. Daylight bulbs such as OTT are the best.
2. Photograph your card with the light shining on it, not shining from behind it. So please don't place your card in front of the window.
3. Get down on eye-level with your card. If you stand over it, it will be difficult to make out the details.
4. Your background is important, so try to showcase your cards. For example I used to stand my cards up on a clean table with the window (light source) to my left, and a piece of 12"x12" patterned paper or plain card-stock in toning colours, propped up against the wall behind the card.
You could create a background that complements your card, like this pumpkin setting. Or a plain black background sets off your work as well, but cameras can find it difficult to manage black. Test and see.
5. Check your photos on your computer before sending/uploading. Make sure they are in focus and evenly lit. You may not be able to see on a small phone screen if a photo is out of focus. Retake if necessary - you can always delete duplicates.