Here’s the story
It can be true that a bigger inventory means a better price. In the case of new vehicles this can be true. They have the increased pressure of inventory turn over. The faster the dealer turns the inventory, the lower the cost of having that inventory (floor plan costs). This is common in the case of new vehicles. Prices on new units can be competitive almost anywhere. What seems like a small inventory for a mega dealer can be huge to the small store. In the case of a new vehicle, do what some of the other posts suggest. Shop price and shop services. Other than price, what services does a dealer offer that you can verify and are meaningful.
In the case of used cars, it is a different story. Having a huge selection of used vehicles is not a sign of a good deal. The larger used inventory requires the dealer to put more work into it and can also require them to pay a little more for units then the next guy. In many cases, huge used car dealers get huge because they pay more and if they pay more, so will you.
Another issue with large used car inventories is they can have a lemon slip through the reconditioning portion of getting vehicles ready for sale. Dealers generally do not add employees in a direct relationship to the number of units they have. Bigger store means bigger responsibility for its people. The one thing a big store does have is selection. More units means more choices. Don’t confuse better selection with better choices or lower prices. In the used car world, bigger volume doesn’t mean a better price and often means a higher price. Every used car must stand on its own and research pays off. Don’t buy a used car until you have checked it’s Carfax, checked the book price on more than one site. NADA, Kelly etc. and looked it over closely. Last but not least, look at the warranty, the dealers reputation and shopped a few places.
Carpenters say “Measure twice, cut once”. I say “Check it out thoroughly and buy once”