Firstly, apologies for the non-reply to your message via our help desk on this.
I think you're conflating a few separate issues here.
1. What constitutes an agreement
In many of the disputes we encounter the "agreement" between buyer and seller doesn't exist or in the absence of clear terms is viewed very by either party. Not to say that is the cause of your experiences, but vague or unclear communication is often at the root of disputes of this nature. e.g. Where a buyer thinks they're first in line or the seller is not entertaining any other offer, but the seller never explicitly agreed to these terms.
Was there a mutual agreement in place with a set of clear terms? e.g. price, method and time of delivery/collection, inclusions/exclusions.
2. Repercussions for buyers/sellers not sticking to agreements
Where issues arise around a sale that took place via Bike Hub or there was a clear agreement in place, the user rating system generally works well to address these issues. Poor ratings are a key signal for would be buyers.
You mention that there are many sellers operating freely with serious allegations / 1 star ratings against them. Provided the allegations are not of unlawful behaviour (in which case more serious sanctions may apply), is it not better to have this visibility of ratings vs. banning a user after X many infractions and risking them creating a fresh profile with no ratings?
3. The ethics of re-selling
There have been numerous discussions over the years around the ethics of reselling. Our view on it is quite simple: while obviously some basic rules and standards must apply, overall the market will self-regulate around these dynamics. As a seller there are pros and cons of selling to an end-user vs. a reseller. Similarly as a buyer there are pros and cons of buying from an end user vs. re-seller.
Relating this to your post: it sounds like you've lost out on a deal or two to a re-seller. Looking purely economically that tells me that either the seller had significantly under priced the item or your offer was significantly lower than true market value (or a mix of both). If your offer was 10% more than the re-seller's offer would the sale have gone to you?
Your note that this is less prevalent when dealing in road bikes is very likely linked to the far lower demand for road bikes in the market. i.e. buyers have fewer offers... less demand to drive up the price and less opportunity for resellers.
Where demand is high your offer will need to be good to beat out the competition.