Who knows the market of your product better? The person who works hard on it for two months and knows it inside and out, or some guy you met anonymously over the Internet who wants $8000 in exchange for half your stock, just to market it?
Think about it, what does he know about the product? Nothing. You'll spend a lot of time explaining it to him in full details and then he markets those details to strangers, parroting your words.
Knowing your product too well is not necessarily an asset when it comes to marketing, especially if you created it yourself. There is often a tendency to overvalue features which were difficult to code rather than features which add real value. Knowing your consumers is far more important.
You don't need "a lot of time explaining it to him in full details" - he doesn't need to know all the details. If you can't explain the product to him in under a minute, you need spend more time thinking about what is important and what not.
Think about it, what does he know about the product? Nothing. You'll spend a lot of time explaining it to him in full details and then he markets those details to strangers, parroting your words.