Two things to add related to +Buy:
1) The absence of +Buy cards
When there's no +Buy cards, cards like cellar and warehouse become can become key buys in first two rounds. Why? Warehouse and Cellar are less powerful in early game but more powerful later when you are greening, for it trades the copper and greens in your hand into bigger treasure. But when there's no +Buy on the board, there tend to be no "good time" to buy these low cost cards, because you're often with higher $ total where you want to buy Duchy, Gold, or Province.
On another hand, when you see Herbalist or Woodcutter being the only +Buy in the game, they could be worth buying early as well. Getting the extra Estate along with your final Province makes a huge difference in a VP race.
2) Trash-upgrade cards (Expand, Forge, Remodel)
They are like +Buy when you're greening. Instead of having $10 and buy just a province, you can remodel your Silver into a tiebreaking Duchy while buying the Province with $8. This effect is enhanced with cards like Peddler. These cards in general can help you to rush to VPs in late game, but in a game without +Buy, these are even more important for can provide you that tiebreaking edge.
