I was recently asked to supply information for the Amusement and Slot Machines section of next year's Miller's Collectables Price Guide. I took the opportunity to quiz their policy on valuations and pointed out that in the past I've seen items with prices that reveal not so much a true market value, more what a particular dealer wishes to sell or buy. I suggested it might help to indicate whether a value was an actual sale price or purely speculative.
I doubt they'll adopt this approach given the extra effort required to verify auction results etc., but they have requested that dealers include items they have recently sold or have in stock, rather than many impossibly rare examples that are very unlikely to be obtainable. In this spirit, I listed only machines I either bought or sold in the last couple of years with values based strictly on actual sales.