I don't think it's a bad as everyone perceives, just a little different. 5 years ago it was fairly hard to find a decent VMX bike but when the Aussie dollar was buying $1.15 US, all of a sudden there was a big influx of VMX bikes bought in by some guys trying to cash in on what was then a fairly exclusive market. Unfortunately the market became flooded, now instead of maybe 1 or 2 '79 Honda CR250RZ's (for example) to choose from, buyers suddenly had a dozen or more to choose from. There's a thousand or more VMX bikes available right now here in Australia without considering the world wide market it is today. Garden variety bikes, Japanese high production models ( eg: Wasps YZ465) are a hard sell for decent money as a result. However if you have something at least a little rare or out of the ordinary they can still command the money they deserve. The trick is to pick the right bikes & tell the truth when selling them. While a lot of sellers don't tell outright lies, they rarely volunteer information. Buyers should ask the hard questions as well. New buyers to the sport who have never restored bike generally have no idea of what is required to do it properly & can't understand why a 35 year old bike is more expensive than a 1 year old 4 stroke MX bike.
The moral of the story is to roll with the times & adjust accordingly.
K