"seldom" is still used by educated English-speaking people. I would suggest that "rarely" is stronger than "seldom".
I used "rarely" because "rzadko" starts with R.
"purchase" is another word for buy.
"They seldom purchase cheese."
Added "purchase", "seldom" worked already.
What do you use more in English, rarely or seldom?
Not a native, but from what I saw in the comments, 'rarely' definitely wins.
Definitely (native here, as you know). Unless you're talking about which was more common in 1800, when seldom won by a landslide. Your useless fact for the day. :)
These days seldom is less common than rarely and has a stronger, more emphatic nuance. "They seldom buy cheese" would usually imply that this fact is a big deal.