Possible manipulation of user transactions by miners in a permissionless blockchain systems is a growing concern. This problem is a pervasive and systemic issue, known as Miner Extractable Value (MEV), incurs highs costs on users of decentralised applications. Furthermore, transaction manipulations create other issues in blockchain systems such as congestion, higher fees, and system instability. Detecting transaction manipulations is difficult, even though it is known that they originate from the pre-consensus phase of transaction selection for a block building, at the base layer of blockchain protocols. In this paper we summarize known transaction manipulation attacks. We then present LØ, an accountable base layer protocol specifically designed to detect and mitigate transaction manipulations. LØ is built around accurate detection of transaction manipulations and assignment of blame at the granularity of a single mining node. LØ forces miners to log all the transactions they receive into a secure mempool data structure and to process them in a verifiable manner. Overall, LØ quickly and efficiently detects reordering, injection or censorship attempts. Our performance evaluation shows that LØ is also practical and only introduces a marginal performance overhead.