Yes it is against the rules.
If you want to sell then you MUST get a vendor account. There is no way around that. And the only people that will PM you with out one are people that are going to scam you.... as no reputable vendor would deal with you. And reputable buyer would not either..though some buyers would if your prices are low enough
A vendor would never take anything in trade.... as by giving you an address to ship something to them would be telling you where they live and who they are. And you couldn't be a drop shipper either. As that would not work for a lot of reasons.
There is a lot more to vending then just buying the vendor account. But if you spend enough time and search around you could possibly pull it off. You need more then just BTC to open an account.
You know how this place works and you would normally get hammered in here. But I think you really didn't know and I wanted to help you out with what you would need to do.
Do a lot of research, take your time, and figure out everything you would need to get an account up and running. If you don't start up too fast you can do it. Just figure out everything you will need and really spend time reading as many threads as you can. There you will be able to find out how everything works and read about some of the vendors mistakes so you do not make the same ones. I know it seems a lot of the answers are not here but they are. You just need to look hard.
Read the "How does it work" page on the bottom of the SR home page. That's a good place to start. And if you have any questions, that are smart, good ones, then shoot LIMETLESS a pm. The guy knows everything you would need to know about SR and everything about vending. And I know he would help you out.
Good luck to you.