Are the miners to decide if a consensus rule is accepted or not? Or are the buyers in power? I get the impression that every time a consensus rule change is suggested then people assume that the miners/mining pools have the last word if this rule accepted or not. But isn't it up to the buyers?

Take the blocksize limit for example: A lot has been talked about which pool would go for which approach regarding the blocksize. Now let's assume one part of the miners would got for approach "big blocks" and the other would go for "small blocks". This would result in a fork of the blockchain: Bitcoin would essentially be split into two currencies: BigBlockBitcoin and SmallBlockBitcoin.

If I where to buy some Bitcoins after such an event, then I'd expect Bitcoin exchanges to offer me these two coin types. I would be able to buy the coins mined under the rules I like, it would not matter if they where mined by 90% of the miners world-wide, or by 10%.

Eventually one approach would lose traction and the coins value. So isn't it ultimately up to the buyers to decide if a coin is successful or not?
Yes. Even more direct: owners of Bitcoins before the fork will suddenly own both types of coins after the fork and can start selling the side they don't want.

However, in general, this is a very messy and damaging way of deciding consensus: in times of uncertainty a lot of people will decide to move to USD to sit out the storm (causing both sides to drop in price). Another fact is that a lot of people do not follow all the day to day drama and will not notice the fork until they're too late. Those people will take the largest losses.

Those things combined make it a further red flag for people considering investing in Bitcoin in the first place: why invest in something that radically changes course every x years through dramatic and damaging civil wars? Where you could be the one losing out if you blink your eyes. A currency needs stability to become trustworthy.

Also: Miners do have to make a choice fairly quickly after a fork, if they mine the wrong side, they will lose a lot of money. And yes users should not want to stay on a 10% mined chain very long: 10% security means that a 51% attack is incredible easy, so that coin would not have enough security to really survive.

All in all it's much better if consensus is reached in a peaceful manner. Luckily the long term goals of both miners and devs are fairly well aligned with the long term goals of users, so those coming to a consensus says a lot. As well as that users (as a group) have a lot of influence over what devs do or don't do (as is the nature of open source), as well as pressure over miners.