I'll preface this post by saying again, the agent should be fired and possibly lose their license for listing the property on the MLS and representing it as being owned by themselves or their investment group.
With that said, I think the OP is way off on a couple of points he is trying to make. First, the wholesaler saw an opportunity with an underpriced asset and jumped on it. If your buyer saw it at the same price as the wholesaler, would they have emailed the listing agent and insisted on paying $75k more because that's what it was worth to them? The wholesaler did a lot wrong here, but contracting the property at the price they did wasn't one of them. They went wrong when they tried to represent themselves as the CURRENT owner. Had they just disclosed they have it under contract and are wholesaling it, it probably wouldn't have been a big deal for most logical investors.
Second, the argument that wholesaling is wrong because they aren't adding any value to the property and stealing equity seems flawed to me. When I make improvements on a property that I buy, I am adding value for myself, not the former owner. If MY profit drops a bit because I paid the wholesaler a fee then that hurts my bottom line, not the former owner. Here is a real life example of my point:
Seller approaches me with a property they want to sell and are asking $X. I run my comps, take a look at the property and decide it's a good deal so I agree to the price.
Scenario 1- I spend $50k and do a rehab, list the property and make a nice profit
Scenario 2- I spend $30k to make the property a rental, find great tenants and cash flow from day 1
Scenario 3- I close on the property, list it on the MLS as-is, make a profit and move to the next deal
Scenario 4- I wholesale the deal to a cash buyer, make a small profit and keep looking for deals
Each scenario is different and each has a different exit strategy, but seller is still receiving $X for each and every outcome. So saying by wholesaling deals you're stealing equity from a seller because you aren't adding any value to the property doesn't hold up for me. You only add value for yourself, it doesn't benefit the seller in any way whatsoever.
Third, the OP failed his buyer big time as an agent. You had a buyer with a lot of interest on the property, all you had to do was keep an eye on it and he would've not only got the property but he would've got a great deal on it. You openly said that the deal had been on the market for a while and you missed it; that's your job! I am not sure about your MLS, but on mine it takes me about two minutes to setup property alerts so the second something hits the MLS I get notified. If you really wanted to go above and beyond you could've checked county records weekly, found out when the bank took the property back, cross searched the specific bank with other properties and found the handful of agents they use to list their properties, reached out to each one and said "hey, if you get this asset can you call me so we can put in a very strong cash offer the day it gets listed?". But you didn't do any of this, you let the deal go to someone else and now you're pissed. You have every right to be upset at the wholesaler but man, I would be super disappointed with myself if I was you. I would use this situation as a way to improve yourself as an agent.