library(httr)
body <- c('search[filter_enum_make]' = 'alfa-romeo',
          'search[dist]' = '5',
          'search[category_id]' = '29')
r <- POST("https://www.otomoto.pl/ajax/search/list/",
          add_headers(headers = c('User-Agent' = 'Mozilla/5.0')),
          body = body)

print(r)