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.

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