Here is a trick

To find out if a dealer will rip you off, take a few coins that you know the value of, and maybe a couple cleaned or altered coins, and see what they offer you. If they are fair, then maybe they are worth dealing with. If they try to rip you off, then quietly pick up your marbles, and mark the dude off your list of reputable dealers. I did that with 3 dealers, all 3 tried to rip me off. Offered me $200.00 for a ag-3 1916-D Merc Dime in PCGS holder, and offered $5.00 each for NGC ag3 1921&1921-d Merc dimes. Also offered triple face value for PDS 1946 mint set in NGC holders. none are less than MS63. So test the dealers you may want to deal with, or just avoid them if you suspect foul play.