How can you tell if an unrestored Wurlitzer that is listed online is worth buying?

There is a short answer and a long answer. First, the short answer: 

Just about every Wurly on earth is worth buying if you like it and it is in your budget. There are almost no problems with a Wurlitzer that are impossible to fix, and all of the consumable parts can and should be replaced anyway. Once you replace the consumable parts and fix the miscellaneous issues, your Wurlitzer will play amazingly. It is the best instrument on earth and you will be so happy that you took the time and the trouble to get one. 

Careful readers may have noticed a couple of phrases: just about, almost. Those phrases are key to the rest of this guide. 

Since Wurlitzer electronic pianos have been out of production for around 40 years now, the most reliable place to find them is on a selling platform like Reverb, eBay, or Facebook Marketplace. Generally, the seller is not a professional restorationist or even a flipper, but an individual who happens to have a keyboard and would like to sell it for money. Because we live in a capitalist society, one would think that many people would be familiar with the basic strategy of exchanging money for goods. This is not necessarily the case.