Like the others say, it's not a scam. But it's a pretty shaky deal, more along the lines of what was going on a couple of years ago. I could be wrong, but it sounds as if the buyers have virtually no cash, and they're stretched to the limit on what they can pay. No cash--because they're looking for $50,000 from you. And stretched to the limit because they rejected your counter of $500,000. (Did the agent present a pre-qualification letter from the buyers?)
And, as others note, your house has to appraise for the sales price. In today's market, lenders (and thus appraisers) aren't as generous as they were a couple of years ago, especially when prices were rising month by month. If you've got a good agent, your house is probably priced pretty close to the comps. Could be a stretch to appraise for $10,000 more...though you'd know better than I.
And the problems with these types of transactions--buyers strapped for cash, buying the most house they can afford, hoping the property will appraise for more than it was even listed for--is that they're more prone to glitches on the way to a sale. Not to say that it'd fall through, but there's more of a risk that it might.
Consult with your own agent. And examine/reexamine your own situation and motivation. This might be an acceptable deal for you. Or it might not. Depends on your personal circumstances and needs.
Hope that helps.