I am not sure if you know it yet but apparently there is no marketplace yet that can be considered an alternative to Flippa in terms of the activities to trading websites.
However I’ll list down a few Flippa alternatives that may help you if you have a problem to list at Flippa.
1) Webmaster forums or related specific niches.Its free to list at most forums however you do tend to get underpriced for most of the times. This may due to the openness between forum members compared to the Flippa’s professional feel. Some popular ones are like DP Forum, DNF, Namepros, webhostingtalk etc
2) Domain marketplace at registrar like dynadot.com, goodaddy.com and moniker.
3) Contact websites that would prefer your website as a complement to their websites. Confused? Aka your current competitor website. Most competitor would actually be more interested to in your site as they will want to control the niche and what better way by eating all the competition
4) Online ads website like Ebay.com, Craigslist, Gumtree etc
I’ll continue to update this post from time to time.
I know a few more but need more info to add them in this post.