You could keep the rims and buy tires when on sale. And swap 'em on as needed. Or just keep the old tires and swap on as needed. It's nice to be able to drop rims off at a shop and have them put on tires, without taking the trailer too. Drop the wheels off one night, pick up the next (I've done that a couple of times with a local garage, very convenient for the both of us).
I had some horribly cracked utility trailer tires (on rims) that I wanted to get rid of. Put them on the curb with "free" on them as it would have cost to take to the transfer station. Gone in a day.