I am a record vendor, and I will agree that there is a wide range in the quality of records at the shows. I see no problem with that as long as they're priced according to their condition. Not everyone wants or needs a pristine Near Mint copy.
On the other hand, there are those unique, rare items that command a higher price, which brings me to your complaint that you couldn't sell your records for their true value. I shouldn't need to tell you that a dealer can't pay you book price for your records.
What source did you use in determining that your records are worth over $150? You say they are Mint, but that is not a realistic grade for any record that has been opened or played. Even sealed records usually have some cover flaws if they have any age on them. A Mint record is VERY rare.
The Goldmine pricing guides are considered the standard, and the price is based on a condition of Near Mint. This would be a record that looks as if it was just opened. Very Good + is the next step down. This is a record that looks Near Mint except for a few minor flaws; maybe some very light scuffing or a very light scratch that does not affect play. These are generally considered to be worth half the Near Mint value. Prices for lesser records take a nose-dive from there.
The other thing to consider are the different versions of a record. You may have a record that looks just like the one that the book says is worth $100, but is really only worth $10. The price guides can't cover all the many variables. It gets really crazy when you try to price Beatles records. There are over 400 different versions of Sgt. Peppers alone! A website called Discogs is a good source of info on the different versions of a record with ballpark pricing.
I make sure the stuff I take to the shows is at least Very Good. Most of my records are pretty mainstream stuff, and the vast majority of them are priced at $3 or $4.
