['countryOfSeller.code=DE', 'countryOfSeller.code=RU', 'countryOfOrigin.code=RU', 'countryOfOrigin.code=FR']
let values = ['countryOfSeller.code=DE', 'countryOfSeller.code=RU', 'countryOfOrigin.code=RU', 'countryOfOrigin.code=FR'];
let newArr = Object.entries(values.reduce((acc, n) => (
n = n.match(/(.*?)=(.*?)/),
(acc[n[1]] ??= []).push(n[2]),
acc
), {})).map(n => `${n[0]}=${n[1].join('|')}`);
console.log(newArr);
["countryOfSeller.code=|","countryOfOrigin.code=|"]