When I buy an old kit from eBay

Lots of variables, so "it depends".

Slight warp on a long flat piece can sometimes be straightened just be hand.   Worse bend on a shorter piece sometimes benefits from a bit of heat (hair dryer on low, not fire).   

If it is a bigger part and only a small area is distorted, I often assemble as it should be and deal with the distortion with sanding/filler depending on if the defect is a negative or positive step.

I have a Steinel temperature controlled heat gun that I can set for 210 degrees and straighten out pretty much any warped model part.

Yes, the gentle application of low heat will fix many warpage problems. Take your time and work carefully and you can correct some pretty bad stuff.

I've saved a few warped pieces by setting in a bowl of hot water. After a minute or two I put them on the bench and depending on the size I clamp them to shape or weigh them down with various weights until the part cools. 

Hope that this helps.

Stay safe.

Jim 

If an airplane wing is warped, I use this trick.

I apply glue to the upper and lower surfaces of the wing and clamp an Architect's Scale to the wing.  The scale is super stiff and you can buy it at a craft store. After the glue cures, the wing is straight.

JohnnyK

If an airplane wing is warped, I use this trick.

 

I apply glue to the upper and lower surfaces of the wing and clamp an Architect's Scale to the wing.  The scale is super stiff and you can buy it at a craft store. After the glue cures, the wing is straight.

 

It works all the time.