Everyone who buys guns on a regular basis has I'm sure a story about paying a little too much for something that caught their eye. That's the advantage the LGS has on you... without ready resources to ascertain the true market, anyone can get unduly influenced by a smart salesperson. People laugh when I carry the Blue Book around at shows, but at least it's something objective to hopefully keep you from getting taken too badly. I also use my phone to tap the Colt serial number lookup, etc.
By the way, a 98% Model 28 lists for $400 in the 1913 BB, but IMHO that is way too low. I wouldn't consider anything less than $600 for mine, were I inclined to sell. In any case, even a $200 mistake isn't catastrophic. The market will catch up.