If the simplified tax system, stupid forget the concept of VAT, when you purchase paying the full price (Including VAT, if those who you sell it to in turn expose), you do everything possible to sell without VAT.
If we talk about "easier" everything is simple.
Bought for 100 rubles + 18 rubles VAT
Sold for 150 rubles + 27 rubles VAT
18 rubles you should state, 27 so that when you have state total you'll pay in the budget of 9 rubles.
If suddenly happen so that you will buy for 150 + 27 VAT and sell for 100 + 18 VAT (well, it so happened) that the budget needs 9 rubles. Of course the tax will assign you a Desk audit, of course will be for anyone to make excuses, but in principle these 9 is to his checking account, though usually not worried and read the following tax periods.