Most agents will rep a novella, but you land an agent with a
novel. That's where the money is for them.
Novellas and short stories usually sell for .06-.08 a word, and 15% of that usually isn't worth their time.
You won't find a print publisher interested in buying a novella from a new writer.
If you have a good mystery, you can try your luck with magazines like
Hitchcock's or
Ellery Queen, but you have to be
good.
You might be able to place one with an e-house, but the money's not nearly as good, and most do not count as a professional credit.
What is the marketability for novellas like?
The market is terrible for a new writer unless you have erotica to sell to an e-house like Ellora's Cave. I think even they only take novellas from their established writers.
Are they sold in bookstores individually or in collections or both?
Go into a bookstore. See what's on the shelves. Chances are good you will only find collections with stories by well-known, best-selling, established writers.
Are there any novellas that sell in reasonable numbers?
When it's a collection with a number of well-known, best-selling, established writers, yes.
Anthologies and collections generally do not sell well, so a big name in the mix gets them noticed.
Most collections for short works are invitation only. You will rarely ever hear of an open call for submissions these days. That's where an agent can help. She hears who's got an anthology in the works and can let you know what's open.
The collections I edited had all their spots filled before the contracts went out and long before anything was even announced in
Publisher's Lunch.
Sorry to be discouraging, but it's a good idea to put on your game face and take on a full novel.
I used to write short while learning the basics of the craft. As I got my legs under me and more confident I found it easier to fill the pages, rounding things out and polishing. NOW I find it's hard to write in the short form!
Go figure.
Before someone asks... According to SFWA:
Novel: a work of 40,000 words or more (Stress
MORE--most houses want 60-100K words.)
Novella: 17,500 -40,000 words
Novelette: 7,500 - 17,500 words
Short story: 7,500 words or less