Your best bet is to put an ad in the local paper, post on Craigslist, etc. Most cemeteries won't sell them for you, even if you offer a commission. They make more money selling their own plots.
Whatever you do, DO NOT use those on-line "services". For most of them, you pay a hefty fee up front, and they will advertise the plots for you. Whether they sell or not, they keep your money.
It would be great if there were a cemetery plot real estate type business that just charged a commission if the plots sold, but I can't find such a business anywhere. I'd be willing to pay 20 to 25% for such a service.
I suppose you could donate them and take a tax deduction. I don't know about that , though.
I own seven plots in two cities/states worth thousands of dollars, but I have been unable to sell them for years. Guess I'll leave them to my kids and let them worry about it.
Good luck.