You're contriving a situation where the seller doesn't have any other options AND you also don't have any ability to buy it to later sell at a profit (which would give the seller the ability to negotiate a better price than $100 with you while still allowing you a reasonable profit when you sell it).

I agree you can contrive a situation where the best ethical option is to pay the seller $100 for the phone but you really have to work on it (and the situation is pretty contrived to begin with)