If I decide to replace my OEM rims, what do I do with the old ones? Is there a trade in value for them? If so, what is reasonable to expect for 18" Sahara stock rims.

 

I made end tables out of mine Stack em' 2 high, run some bolts thru em' & get some glass cut for the top Had so many compliments I've lost count

 

I keep my stockers for winter tires here. Or sell them on Kijiji or Craig's list, whatever. There is also a for sale forum on the JK site here you would likely be able to unload them on.
Look around and see what other sets are selling for in your area to get an idea of what to ask for them.

 

Sold my 17's with stock tires for 475 through Craigslist. Helped pay for new wheels. I did pull the TPMS sensors before though.

 

I plan to wait until I actually need tires, then just do the whole shebang at once. I'm at 27K and guessing the tires have another 6-8K. Hope so anyway.

 

Even down to the wear bars you will find people on craigslist willing to buy them.