Letting users have the choice of currency is in the spirit of decentralization. I should, ideally be able to buy in any currency of my choice with a single click conversion being carried out at the backend.

To simplify UI, this could be done with 2 steps:

Step 1: In my Profile Settings, I could shortlist 5-10 currencies (ETH, WETH, MATIC, WMATIC, USDC, AAVE, LINK etc) - Here, you can show the most commonly used currencies first
Step 2: At PoS, it should show me the chosen currencies in a drop down

For the seller:

Step 1: Shortlist the currencies I wish to accept in my Profile Settings
Step2: Set the value in those terms & receive the currency of my choice.

Pegged Value should be independent of choice of Currency. All Swaps should be under the hood