You can publish the identity of a node you are running for fun and/or so others can use data you publish for research, however if you do this, I would use another full node (including possibly an electrum server) to verify transactions sent to you, especially when the other party sends first and when dealing with larger amounts.
Bitcoin Core doesn't really have that behavior. The client must have at least 8 outgoing connections to the network that the clients wants to connect to. Assuming reference client, it is (almost) impossible for you to attack someone via Sybil attack if they can have a choice in whichever nodes that they want to connect to. Unless of course, you manage to get a whole bunch of IP addresses in different IP blocks and force the victim's client to connect to them only.
If you can successfully execute a sybil attack, the cost to find a block that is on top of a recent (but not most recent) block will be roughly the total approximate block reward, so it will not cost much more than 12.5-13 BTC to make your trading partner see a confirmation if you have executed a sybil attack.