I've been approached numerous times to sell photos. My 8x10's have gone for as much as $35 unmatted. I sold printing right to an insurance company on one for $250. Charity auctions though have brought as much as $450 for an 18 x 24 on canvas framed. I had only $70 in that one. It's really kind of all over the board. You might look at some photo galleries, gift shops, etc that sell photos in racks. Remember that they usually double the cost price to come up with a retail price. If you can get into a street market event people will tell you what your work is worth. Be ready to haggle. Good luck.

Just say that to date you have NOT sold a pic but would love to sell them one, or not. Tell them what you would like to sell it for and if they say yes, them you've made a sale!
Everybody that sells starts somewhere. I've never used SM so have no idea what is needed to go through them or what % they take.
I certainly don't agree that the second you sell something it suddenly becomes a job and is no longer fun. Some people love their jobs and would almost do them for free!!
Anfoto has given you some good advice.
I guess the question is more, is it even worth it if it's the ONLY pic you will ever sell if you are going through SM. good luck. ;-)
SS

The best thing you can do is ask the buyer what they want...

If they just want a print to hang on the wall, that's one thing. If they want to use your image on their website or for commercial purposes, that's another thing entirely. There are also stock photo sites that "troll" photo sharing sites like SmugMug looking for contributors.

If it's just a print, then I'd suggest you look what others are charging for similar size and type of image.

If they want to use it online or for commercial purposes, then you license the usage, you don't sell it outright or sell your copyright. Once again, there's a wide variety of costs depending upon the exact usage, length of usage or distribution, exclusivity, right of renewal and more. If being used for these purposes, in many cases if there are recognizable people and/or property in the image you also would need a signed model and/or property release.

If it's a stock photography company soliciting you, proceed very, very carefully. Some give you virtually nothing for your images and/or try to get you to give them exclusivity.

So, first thing you should do is contact the "client" and ask them what they are wanting to do. Then come back here with more details and maybe we can give you more and better guidance.