A few thoughts. Single crown varieties should be, well, single crowned! If you're buying a starter plant and can see a picture of it, such as you might on eBay, and it has more than one crown the grower is obviously not paying close attention to their inventory. As such, I'd suggest skipping that particular purchase.
Healthy, clean and a shiny appearance. If the starter plant doesn't have an immediate heir of health and vigor, skip it!
If the crown (main growing point) is not clean and clearly visible, skip it! If the leaves aren't crisp and growing horizontally, skip it.
Starter plants of trailers (as seen below) don't necessarily have to have multiple crowns when you purchase them, but they should have a nice shape to them. The foliage color should also be clean, rich and healthy looking and void of excessive scarring or damage. If the foliage is curling, twisted or otherwise distorted, skip it!
Remember, if you're in doubt about the health of the starter plant you're about to purchase, error on the side of caution and pass on that particular purchase.
Don't forget to check out my eBay African Violet auctions by visiting my eBay store. Click here and I'll redirect to my store.