The article sounds like it's geared toward relationships with pawn shop owners or thrift store owners. People who don't necessarily follow the value of games or their store doesn't focus on games.
I generally find it's just best not to annoy the people who run the place and try to be a gracious customer who doesn't demand too much.
I used to frequent two different places and would have different results based on the person working at the time. One place would put higher prices on stuff like Mario, but let stuff like Mega Man X3 slip by for $3. I usually just look through their stock, comment on how I enjoy shopping there because their prices are fair and buy a bunch when I can. Several times they have said "hey, I just got these and haven't put them out yet" which has led to some nice deals on Mega Man X2 and an Aero the Acrobat 2 for SNES.
The other pawn shop is more hit or miss. They have two locations, one run by mother one by daughter. The mother would always work out a deal, and one of her workers frequented the store I worked at, so deals were to be had. The daughter would refuse to deal. I would pick up a stack of 5 or so games and usually say "can you make me a deal?" The mother, and other workers would cut a few bucks off... the daughter would say "nope, they are already on sale" and refuse to give any sort of discount. I usually just walk out while she is there (her store closed and they are down to one)
