Investment in mining hardware whose value is tied to a speculative asset without fundamental value beyond utility.
None of the current implementations can scale, so even their utility is questionable. There is a way to make a scalable trust-less digital currency by using a mutable leder, but none of the current coins that I know of are heading that way. The main difficulty will be how to distribute ledger space in a way that cannot be monopolized by the richest. That is a simple technical problem to solve though, because storage capacity will eventually surpass the ratio of richest to average wealth. Then you can allocate ledger space in exchange for an affordable deposit, redeemable when the balance is emptied.
> (4.5) Alternative Stores. You may not use Google Play to distribute or make available any Product which has a purpose that facilitates the distribution of software applications and games for use on Android devices outside of Google Play.
You have no chance of bootstrapping an alternative store outside of Google Play. Sideloading APKs is not how users get software on Android.
Amazon failed hard with their attempt, and they pumped tons of money into it by paying developers to give away their apps for free; that, and they shipped hardware that has their store preloaded. Still didn't even make a dent.
Trust level. If you see a seller advertise 1 teddybear for sale, and you know they burned 3 teddybears worth of coins just to offer you that first teddybear, you can be pretty certain they're not going to run off with your money.
On the other hand, if a new seller didn't spend anything, they could run off with your coins on the first transaction.
This isn't just theoretical; it's the central problem in most darknet markets.
And if they are participating just to build a sufficient level of trust before doing a runner, then you are saying there must be a ratio of "trust to stake" - maybe 1:1 at first maybe increasing - it would be interesting to know what market makers in stock exchanges run at - 1:10 or more?