How do you plan to keep the price the same even when as you said "there could be millions of sellers and no buyers". Wouldn't people just exchange them for less outside of the in-wallet exchange?
I know if I had a coin that was worth x-amount but I couldn't sell it for x-amount I would happily take 80% of its value OTC, or less even.
I've always thought that the "Terra" idea (not to be confused with Terracoin) is one of the most innovative currency ideas, but it has it's own problems too.
The best I can think of is a wallet lock that will only release the coins at the exchange rate. Even if this was done, you could still get around it by purchasing an item for more than the exchange rate, e.g. a $500 item could be marked up to $1000 for the sale.
The only reason fiat currencies and cryptocurrencies work is because of faith, faith that you will be able to go out and buy property, food and fuel. By fixing the price to an asset the faith in the coin is solved. There will always be someone who will sell for less, as long as there are retail stores and people willing to accept the coin at the exchange rate, this should self correct. If it takes a considerable amount of technical knowledge/effort to get around the exchange rate, then the majority of users should still accept the exchange rate.
Maybe another option would be each wallet is it's own blockchain that requires low energy mining to acquire coins? Using an 2-4 core CPU would require 2-5 years to mine the coins. This would help to reduce a flood of coins being sold OTC.
I said "million sellers and no buyers but the price remains fixed" because I don't like the way other pegged coins have so much variance from the base asset, bitUSD is currently at $1.17?
I'm not sure what you mean by "Terra"?