The usual places are: http://flippa.com/, http://empireflippers.com/ or https://feinternational.com.
I've also just posted something on HN: https://news.ycombinator.com/item?id=19781447
Hey, I’d be interested in buying the web app (if it’s the one I think it is). Feel free to drop me an email (in profile).
^^Interested in knowing this answer
Calculate the Seller's Discretionary Earnings (SDE) and usually for a web app biz like this, you can get 1.5-3X the multiple of SDE. SDE is generally the profit after all expenses plus seller's portion of salary/bonus etc. So lets say the total revenue with 22 paying customers is $10,000/Year. Your cost to run this is $2500/Year and you also pay yourself a salary+payroll tax of say $5000/Year.
SDE = ($10000 - ($2500+$5000)) + $5000 = $8500
So you may get anywhere from $12,750 to $25,500.
Since your business is only 1.5 years old, getting a 3X multiple will be very hard. Most likely, you will settle between 1.5-2x.
Depending on the actual revenue (since you didn't disclose), you can sell it on sites like flippa.com, empireflippers.com, sideprojectors.com, 1kprojects.com etc. It all depends on the revenue.