what’s the current best way to buy and sell compute onchain? (if any)

Not sure about “current best” but Render is interesting. Here is a podcast I recorded with the founder if you’re curious to learn more:

Not sure about “current best” but Render is interesting. Here is a podcast I recorded with the founder if you’re curious to learn more: https://podcasts.apple.com/us/podcast/pod-of-jake/id1525087226?i=1000635665799

Someone asked me about crypto-backed loans on X; which this seems well suited for... even better is loan for leases [don't buy GPUs]

Platforms like Akash Network, Golem, or Render Network allow users to buy and sell decentralized compute power directly on blockchain networks.

Isn't eth kinda owning compute?

Or are you talking about renting out GPUs? Render and Nosana are ones I know of on Solana

Hyperbolic 
Spheron
Aethir 
Flock for decentralized model training  
Truffle is building a network of local machines