What a run actually looks like
A careless autobuyer searches as fast as the market will answer, all day, from one account. That is the behaviour EA looks for, and it is why people end up restricted. A sensible one looks more like a person who is quick but human: a gap between searches that is never the same twice, a break every so often, and a session that ends rather than running all night.
FC AutoBuyer is built that way by default. Search delays are randomised inside the preset you pick, breaks are scheduled, and a run has a time limit, a search limit and a limit on how many cards it buys. Nothing stops you turning the speed up, but the fast presets are meant for short bursts, and the extension says so when you pick one.
When it stops by itself
Stopping early is usually the right move. A captcha means EA wants to see a human, a soft ban means your searches are being throttled, and a run of errors usually means the Web App has changed under it. In all three cases FC AutoBuyer ends the run and tells you, rather than hammering away at a market that has stopped answering. Our guide to the transfer market soft ban explains what triggers one and how long it lasts.
What it never does
It never asks for your EA password, never logs in for you, and never touches your account outside the Web App tab you have open. It does not sell coins, buy coins or talk to any coin service. If you want the detail, the privacy policy sets out exactly what is stored, which is almost nothing, and all of it on your own computer.
The risk that remains
Being honest about it: using any autobuyer breaks EA's terms for Ultimate Team, and a ban is possible however carefully you run it. Keeping runs short, the speed sensible and the account one you can afford to lose is the only real protection. There is more in how to avoid a ban when using an autobuyer, protecting your account while trading and our terms.