That's a ridiculous counter argument. If I buy a bunch of towels/etc. from an online merchant, I have every reason to believe they will not use any information they gather to publicly embarrass me or otherwise do me ill will. Their business relies on them not abusing their customers. If they showed any indication that they aren't trustworthy, I wouldn't do business with them.
On the other hand, I don't have any reason to trust someone on the internet requesting my credit card records in exchange for the privilege of being publicly embarrassed when they post it on the internet. The argument is never "I've got nothing to hide", the argument is that I have no reason to hide my Amazon purchases from Amazon.
Most of your examples are either non-issues or not realistic. My credit card company does have a right to see how much and where I'm spending - I'm borrowing their money when I make a purchase. I don't know of any service I use that sends my porn history to my boss (unless I decided to do it at work on computers owned by my employer). I don't know of any supermarket discount program that prevents you from shopping elsewhere - every rewards program I've ever seen is structured to encourage you to shop with them more, not punish you for shopping elsewhere. My wife has every right to scrutinize our bank account - if I didn't trust her I wouldn't have married her. If she finds out what her birthday present was, that's her loss for ruining the surprise, not mine. My annoying jealous nephew can get over himself - if he wants to come on trips with me then he needs to be less annoying.
If I don't want any of these things posted publicly on the internet, I a) won't post them publicly on the internet, and b) won't share the information with anyone that I don't trust to refrain from posting it publicly to the internet. Anonymity is not the same as privacy - privacy comes from keeping personal information to yourself.
So, to reiterate the grandparent poster asked, why does a distributed marketplace intended for legal trade need to run on Tor? Do you need anonymity if you're selling towels?