Syndr0m, I hope you don't mind if I reply to your question even if I'm not funkyworms and maybe not as knowledgeable as him. The person who is offering you VPN services can of course sniff everything you send via his VPN server. You probably already know that if you speak of sniffing packets. Plus, he can reach your computer directly, because your computer and his computer are on the same virtual network, and he knows your IP address on this network (because he's the one who gave you the IP address). It may even be possible for him to execute programs on your computer remotely (see
this), although the probability that he's able to do so is very low. Unless you download some program from the internet via his VPN using a non-encrypted protocol (http, bittorrent, ftp, etc). Then it's easy for him to execute whatever he wants on your computer.
Of course, you can choose to use his VPN to route encrypted traffic only (let's say some poker software), and you can use some firewall to block incoming connections. But still, you can't be sure that the guy isn't smart enough to manage to exploit your computer.
He could even detect when you're playing poker, and if he knows your screen name at the poker site, then he could play against you and disconnect you (either by shutting down the VPN or by flooding your internet connection) so he can win a big pot against you. But if you don't use this forum securely as I said earlier in this thread, then anyone who knows your screen name can already do this do you.
Also, if you intend to play poker via this guy's VPN, then you must ask him if other people will do the same, with the same outgoing IP address, because it could trigger an alert at the poker room if 10 guys are playing with the same IP address.
In my opinion, one should avoid to use VPN services from individuals or from company specialized in VPN. What I would do if I wanted to use a VPN is to rent a dedicated server with a dedicated IP address from a very big hosting company, and install my own VPN server on it. The only problem would be that my IP address could seem suspicious to some poker room, as it would not be an IP address from a cable/dsl connection (maybe you have the same problem with the guy who is offering you VPN services).