Bike stores don't always take trade-ins, so if they paid you anything for your old bike, they saved you the trouble of selling it.
Bike stores do often sell below retail, but in free market exchanges the process is:
Offer
(possible haggling goes here)
Acceptance or rejection of offer/counter-offer
What you got was fair, whether it is more than everyone else in the world pays or not.
Don't second guess yourself over whether you paid an extra $30 after the fact, it isn't worth it...
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Slow Ride Cyclists of NEPA
People do not seem to realize that their opinion of the world is also a confession of character.
- Ralph Waldo Emerson