Trying to sell burial plots. Any ideas on how we can facilitate this?

I would donate them.
You would get a write off of the value of the donation.
Someone would get a plot that they need.
I would contact a Hospice in your area and see if they would want them. They could use them for a family in need or they might be used as an item in a Fund Raising Silent Auction.
The Hospice I volunteer at has a high number of Pediatric Patients and I am sure many of them do not have extra funds for funeral arrangements or plots.

Have you tried selling them back to the cemetery. That's what we did.

My suggestion: I would have named the location of the plots on here more clearly. For example; "Getting rid of 2 plots located in Jackson, Mississippi any takers?" Then list your contact information. I would put it on EVERY social media site. Good luck

When my husband passed I had to sell our plot because I needed the money. My husband and I decided to go with cremation. Not knowing how to do it I contacted the cemetery. Was able to list the location of the plot in the paper and sell it at what we paid for it (a lot less than the current price) to someone who did not have arrangements made. It worked out for me. I needed the money and they needed the plot. Since we had also paid for the grave marker the cemetery let me keep that.

What about contacting area funeral homes, perhaps offering a percentage? People come to the forum all the time worried about funeral costs, I imagine people would snap them up if they only knew about them.

Check with the Funeral Home or wherever they purchased it from and ask them to see it for you and ask how much they would charge.

You check with the cemetery owner and see if they'll buy them back. That's what I did with two single plots we have in our family that were intended for tho husbands of two aunts who never ended up marrying.
For what it's worth, in my case, the answer was no.

They didn't want any single plots, but you might have better luck with the pair.

My brother tried selling back to the cemetery. He told me they couldn't
say "No" fast enough or often enough. I will add that the location is in the southern US. It would seem that there would be more of a demand in that area, but obviously not.

Sometimes churches assist members with burials. You might check with churches in their old home town to see if they would be interested in a reduced rate.

One of the problems with cemeteries buying them back is that the grief counselors won’t get paid unless it is a “new” sale and that’s often the contact person. It is a tough job they have. They might have authority to sell but not to buy lots.
If the lots are located in one of the chains, try contacting the corporate office to see if that level has an interest and make sure you are speaking with someone who can actually say yes.

You could also see who is buried near the lots to see if you can contact extended family who might want to be buried near their mom or dad.

You can search on FindAGrave and possibly read obits to see who those survivors might be.
I am surprised that the arrangements are transferrable.

Many of the National cemeteries are running short of space. It’s good you already have your dads space reserved by mom.

Some cemetaries participate in an exchange program where the purchase of plots in one location can be exchanged for those in another town. There is some affiliation they belong to. Perhaps the plots your parents bought could be exchanged to benefit someone else in the family who has not made final arrangements.