The thing you should understand about hypotheticals is they're not a substitute for the equal sign. When I present you with the hypothetical "why make murder illegal," I do not mean to suggest that !(proscribing account sales) = !(proscribing murder). There are, indeed, differences.
The text you have quoted was a response to TL;DR: "account sales are a necessary evil, better than they happen here than elsewhere."
If you feel that account sales are desirable, like gun sales in your example, feel free to explain why.
That said, a forum created to advance Bitcoin is not necessarily the best place to sell guns.
Or to double as an online scam bazaar.
Or as a casino.
Or as an MLM ponzi promotion vehicle.
Or a banner ad platform.
If you & thermos didn't, in all your wisdom, endow it with all these trappings of petty scumbaggery, account sales would not be an issue.
See how you can solve a problem by thinking outside the box?
Buying account increases forum traffic, because of signature campaigns, which inturn benefit signature campaign owners, participants and the sites. Some scams along the way, well just Caveat emptor, if you know accounts are being bought you'll get more suspicious and which will prevent you from other scammers as well.