You agent - or potential agent should help you with that. Some agents just need a covering letter with a short precis. If you have an interested agent, ask advice from him/her. After all, that is one of the things they get paid for. Good luck.
It might be an idea for you to send a short summary of the article to the magazine of your choice, outlining the content without giving too much away. Possibly the first page or couple of paragraphs. Enough to grab interest and make them want to read more. Ask if they are interested.
On the safety level, so long as you have obvious proof that you are the original author - notes, hand written copy or something like that - then clearly you own the copyright and should be quite safe.
I know how you feel about involving an agent, but the advice is always to do just that! It would be worth it if it would result in further sales of your work. I think agents and publishers are always interested to know how much more material is likely to be forthcoming. An agent would know exactly the best publisher to place your work and would give you plenty of advice all down the road.
The essence of your question then, your work would be copyright and you would have protection. An outline with a brief summary of the entire article (along with a tantalising sample!) will be enough for a buyer to be interested - or not as the case may be!
I do hope this all works for you. Please let us know.