The price often depends on the selling venue, that is where it's being sold.
A gun shop has monthly overhead and needs to pay their expenses such as taxes, insurance, wages, heat etc...
It's not like the guns still belong to the original owner who are often more willing to cash them in for a fair price.
However I doubt that the original owner received $450 in trade for them, even if he had traded them in towards a high end firearm.
Most gun shops want to clear at least $50 per gun if they've invested in it.
But it sounds like that shop wants to clear $100 per gun or more.
Remember that their business is based on the formula of "buy low and sell high".
Considering the price that they're asking, $450 would only be a starting point for opening negotiation.
Even paying $500 wouldn't be too bad if they're both really like new, and could very well still give the shop a net profit of $50 per gun.
However offering $500 out the door would still be better than paying $500 plus the state sales tax.