A woman didn't fufill fasting of the month of Ramadan two years ago, due to pregnancy and breastfeeding, at the time being she is making up the missed Ramadan. What is the ruling of Sharia? And what is due on her?
Whosoever break the fast during Ramadan or didn't fast at all due to health concerns, is obliged to make up the missed fasts whenever she could so long as making up missed Ramadan didn't extend to the coming one, and if next one arrived without fulling fasting the missed one, the ransom is 60 grams for each missed day (Equals 60 piasters to one Dinar for each day). And Allah The Almighty Knows Best.
What should a person who doubted having missed a pillar of prayer do?
All praise be to Allah, The Lord of The Worlds. The prayer of one who doubts having missed one of its pillars is valid, and nothing is due on him/her since having a doubt after offering the act of worship doesn`t undermine its validity. And Allah Knows Best.
Is permissible to buy/sell with non-Muslim?
Yes buying/selling for a Muslim with non-Muslim is permissible as regards what is lawful.
Which is better, to pray at a prayer place (Musala) whereas many people pray in it, or at a near by mosque in which a few people perform prayer?
Praying at the mosque that has a bigger number is better so long as the number of praying people at the other mosque permits performing prayers in congregation. And Allah Knows Best.