Yeah, fishy. They were counting on that scenario (and likely not reporting income).
At the same time, I always have exact change or confirm ahead of time they have change if I’m dealing with someone on Marketplace (or otherwise need to use cash). Very much a done thing to list something for $90 and shrug when you show up with two 50s and expect you to take the ten dollar haircut.
Yeah, check out The $100 Startup by Chris Guillebeau (I haven’t read it, but there was a lot of good info/crossover in his later Side Hustle book which I did read).
He has a podcast which is TBF a bit padded out with advert/self promo, but OTOH is free and does have some good info. He interviews a different person who has started a side hustle and asks questions about things they did right, mistakes they made, etc.
Unsolicited I wouldn’t say there’s any obligation to respond, but I think it’s a nice thing to do; you never know when your paths may cross again.
As a candidate to an actual job posting, something that really stings is when I clearly put a lot of time/effort into a cover letter and get absolutely nothing from the employer. Some HR people will say ignoring unsuccessful applicants is best practice to avoid starting a potential conflict, but IMO it shows a basic lack of decency and I add that business to a personal blacklist of places I won’t patronise or recommend to others.