A probable market is a market that accepts your type of work. This means you must do your background research. For short stories, a great speculative fiction resource is www.ralan.com. (Does anyone have a great general short fiction resource?) For literary agents, a great resource is www.agentquery.com. I've blogged about market before.
A timely fashion is a bit trickier. The industry convention for novels is you may query agents simultaneously for the same work. However, if you get asked for a partial or full manuscript, generally, the agent prefers an exclusive. The industry convention for short stories is NOT to submit the same story simultaneously. This means you can only submit to one market at a time. If you are submitting to a SFWA-approved professional market, for example, I would definitely abide by this rule. If you don't follow industry conventions, you run the risk of offending an editor or agent. That can have a negative impact on your career. Yikes!
Another fly in the ointment is publication. What exactly constitutes publication? If you post part or all of your story on your webpage, is this publication? What about a novel excerpt? What if it's posted on someone else's website or ezine? What if you get compensated? What if you don't? What about rewrites? How much do you have change to make a 'new' story? Regarding this stuff, just be honest and don't try to mislead anyone.
Of course, when the money starts rolling in you have to keep records of it for the tax man. :) I hope you have that problem!
I've tried quite a few things in pursuit of writing records including spreadsheets and databases and, I must admit, none of them work great. Do you have any good tips? If so, please let us know!
The bottom line is: it is important for writers to keep records. Good luck!
Stay tuned as next week we start bragging about the next marvelous issue of Electric Spec!