I disagree with some of the opinions in this thread.
Especially "no-one wants our built kits"
You guys should know that there are just as many "kit-bashers" as there are straight-builders.
And even a straight builder needs a part once in a while, either because they lost or damaged something, or just want something extra.
And then there are those who like to rebuild models.
But what about older, classic, vintage or rare models.
They are selling all the time on ebay and the like.
I easily sold a couple of very old vintage models (built) at a model show.
Especially easier if you have boxes and/or Instruction booklets.
If you really don't hold any value in your old kits, then put them all into a box and sell them as a parts lot.
An interesting collection of kits and/or parts will appeal to any modeler searching online.
And if they are cheap enough, someone will gladly buy the lot.
I'm not even a full time plastic modeler, but I will still stop and look at a sale like that, just to see what treasures might be in the box!
And again, if it's cheap enough, it entices me to look and consider buying.
And when something comes with it's original box and papers...
Granted, newer over produced stuff doesn't hold any value.
But that's where the kit-bashing appeal comes into it.
A bunch of extra tires and wheels...cockpit parts...boat deck accessories...engines for hot rods...
A box of a dozen old car builds for $10 sounds pretty good.
Grab a box, jam as many kits as you can in it...take a photo of the mess.
List all the kits in the box.
If you have them, stuff the folded up boxes and Instruction sheets.
Seal the box, weight it and measure it.
The eBay listing does the rest.
Shipping cost is not your worry, let eBay do that...you only need to take it to the post office when it sells.