There are tons of sites out there that allow you to sell stock vectors and photos. I have sold stock vectors on most of the major ones and here is what I took from the experience.
If you have good, useful stock; it will sell eventually.
Payouts thresholds on these sites can be high, and therefor take a very long time to reach unless you have a huge number of files on offer.
Most require very specific file preparations that can be pretty time consuming.
I still sell my unused client work on stock sites but with a modified approach. Instead of spending lots of time uploading to all of the most popular sites I chose the one that pays artists the highest commission for their work Graphic Leftovers (http://graphicleftovers.com/?ref=benbrush) and focused on it.
Instead of waiting to build up enough sales for a payout I actuality use the revenue from the site to pay for stock art whenever I need it. This eliminates an expense, which makes my other client work more profitable!
I write tutorials
Did you use a cool or interesting technique while making your design?
Write a step by step tutorial explaining it and post it on your blog or look for paying blogs that are looking for guest posts.
I design merchandise
A lot of designs can be easily tweaked to make them work as Posters, t-shirt designs, phone cases etc.
I like to use sites like http://society6.com for this because they handle all of the printing and warehousing, making it perfect as a source of passive income.
I turn them into screen prints or art prints.
Edit your leftover designs into screen prints and art prints you can make at home and sell in an online shop like Etsy.
I built a giveaway pack for my website.
A lot of the time client work can end up being general stuff that anyone could get some use out of. Sets of icons, Photoshop brushes, themes and other items can make awesome giveaway packs to bring traffic to your monetized blog or portfolio website.
I use leftover card designs for my own promo material.
If you made a knock out business card or rack card design for a client, that you didn’t end up using, it can be a huge time saver next time you have a promotion you want to make some marketing materials for.
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If nothing else, it can boost your portfolio. In either case, your business gets a promotional boost.
I tap into the power of affiliate programs.
Write a review or how-to of a product or service that was used in the making of the unused work and post it on a blog with an affiliate link.
More ways to turn unused designs into cash?
None of these on their own will make you rich overnight but if you use them as a whole package in addition to your normal client work they will add a nice bit of extra income without a lot of extra work.
What are some techniques you use to boost your passive income and reuse the leftover client work?