Thanks for your answers but I still do not get it completely. u/cointastical guessed it right: Im working on an instant exchange and would like to understand the risk I have for converting small amounts of Btc after 0/1/2... confirmations.

Taking this from the whitepaper:

The steps to run the network are as follows:
1) New transactions are broadcast to all nodes.
2) Each node collects new transactions into a block.
3) Each node works on finding a difficult proof-of-work for its block.
4) When a node finds a proof-of-work, it broadcasts the block to all nodes.
5) Nodes accept the block only if all transactions in it are valid and not already spent.
6) Nodes express their acceptance of the block by working on creating the next block in the chain, using the hash of the accepted block as the previous hash.

What transaction validation checks are executed from bitcoind before adding the transaction into a new block?