Running some code on a CPU != running a startup. Great you can buy a Dell server on eBay, or you can build a powerful desktop, or rent a VM or get a droplet or scrape on lowendbox. These are not a secret and there is a great reason no one does this other than hobbyists and neckbeards.
You do your testing and it works, then what? You have to deliver scalable reliable systems in production that require identity management, security, backups, resiliency, reliability, various networking services and a million other supporting services and all the systems that come with it. Never-mind actually scaling the application, monitoring it and all the tools, systems and processes needed to run reliable systems in production.
The eBay servers provide you exactly zero of that and you've just wasted time setting up an environment that is a snowflake and doesn't represent reality. Testing on the cloud on exactly the same platform you would use for production has a lot of benefits when you look at value as limited developer time delivering value to customers and the business.
Whilst the $1200 server on eBay might be cheap today, you are entirely missing the hidden cost of lost time when your team of developers costing $M/year are wasting on testing in an environment that doesn't help them find and solve production issues. You don't need many hours of wasted time or downtime to lose all of your so called cost gains.
Optimising for absolute minimum cost is a fools errand that only slows down actually delivering production systems that deliver value to your customers.
Please spend some time thinking bigger about the opportunity cost and value delivery of technology beyond the immediate dollars and cents - it might surprise you.