Why are you assuming there's something wrong with it? I'm not pointing to outliers that are only cheap because they're broken. The average market price for an M1 MacBook in my area is around $350.

I'm seeing a lot of M1 Macbook Airs around £200-250, and Pros in the £300-350 range.

What's wrong with them? The M1 was popular and now people selling them are competing against a lot of other people selling them which suppresses the price. Like it or not, Macs are mainstream and therefore aren't going hold a "magic" high price.

I just searched my area. There were plenty listed as having no issues at that price point. eBay has similar deals. No reason to think that $350 isn’t the market price.

Could be boot locked to the original owner Apple account.