Congratulations! You'll find a lot of love here for the 457. I have the stainless version and it's my 2nd favorite pistol (after my 645).
Attaching a value to these old(er) pistols is hard(ish) and depends greatly on where you live. Black ones in 90% plus go for $350-$425 here in FL...if you can find one. I paid $350 for mine, with 3 mags & no box, but had to drive 2 hours to get it. I've seen a 2tone only once and it was about 6mos ago. They were asking $475 and I don't think they got it, the listing expired.
I'm honestly not sure people know what they're looking at when they SEE a 457 (and most Smith semi's). I'd probably pay $500 if it was seriously LNIB...with the box & swag. YMMV, I'm sure others will chime in shortly.
As for how to field-strip:
With the barrel facing left, you will see the "slide-stop assembly". On the other side you will see where the pin comes through.
On the slide is a notch just forward of the assembly. With the mag out, pull the slide back until the notch is over the front part of the slide-stop and hold it there.
Using your right index finger, you should be able to push the pin straight out a little.



STOP RIGHT THERE



Pull the slide-stop STRAIGHT OUT. Don't let it swing down or you'll scratch the frame.
Remove the barrel and spring like any other semi and go to town. Be advised: That double recoil spring can and will go flying
Upon Re-assembly, you'll have the slide on the frame and there are pins that have to be depressed as the slide moves rear-ward.
With the hammer down, proceed to depress each pin until the slide is over it.
Pull the slide back until that notch is over the hole and insert the pin STRAIGHT IN

.
That procedure works for all 3rd gen S&W semi-autos. You should be able to use any youtube vid for any 3rd gen if you have problems
Hope I helped, enjoy!
Oh, yeah...almost forgot...



























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