No, both PoS and PoW allow everyone to participate in exact proportion to their resources.
No: you can't buy coins of the chain you want to mine if they are not for sale, whatever are your resources.
While you can build how many hashing power you want if you have the resources, and nobody,
nobody, can't stop you.
The difference is quite abysmal.
I should clarify that when I talk about PoS I mean only as a synchronization method. Every coin that uses PoS synchronization also needs some issuance method, and the best known issuance method is PoW. The problem with the PoS coins we see today is that they think that by using PoS they don't need PoW, so they use a broken issuance method instead.
For a proper PoS coin that uses PoW issuance, everyone can participate in "the game" by acquiring hashrate normally and minting new coins.
If you move forward in time past the original distribution, it is true what you say that to participate you need someone to sell you coins. But in practice coins are being sold on the market, so this is only a problem if someone tries to acquire a large amount - and that's not really a problem, since the most likely person to do this is an attacker. Therefore, I consider the difficulty to acquire a large voting power quickly an advantage.
POS has its own very serious problem:
Someone with a stake can spend it to mine any number of forks simultaneously.
In contrast POW can only be created on alternatives if computing capacity is split between them. This forces convergence while POS does not.
If you examine the
designs listed in the wiki, you'll see they're both resilient to this.
In my system, if a stakeholder signs two conflicting blocks, evidence of this is referenced and the voting weight of his address is reset. (Moving to a new address also resets voting weight, until it accumulates weight again).
The same cannot be said about the alts that pass for PoS these days.
Why then, do you think we haven't adopted it yet?
1. It's too big a change.
2. It doesn't work well with merged mining and alternative uses of the blockchain (a la colored coins).