Here is my experience with Flippa.
Had 4 domain listings in total in the last 3 years on their platform, three were $9 listings one was some $240 ish ugrade listing in 2015 which ended up as a total disaster. No surprise, I was trying to sell a shitty handreg at beginning of my domaining career, and there is no upgrade on this planet that could sell a smoking fresh bad quality hand regged domain name with better results. Rookie mistake.
For the second time I've listed a premium name I spent $580 on Godaddy auctions, and tried to sell on Flippa with the $9 listing half year later. I lost $500 this time. The upgrade I paid for the first listing should be paid here, and who knows how the story ends.
Third time, (last year December) sold a name for $230 (backordered for $69 one and half year prior) with the basic $9 listing. This name was a category killer name in it's niche, so even though I approached Flippa with caution this time, I was somewhat confident that it will be a profitable auction. I've invited end users & some domainers from Namepros, and most importantly I've set the auction's time duration to 3 weeks. So I'm sure if I try to sell it in a week long auction without inviting people, it would ended up as a tragedy again.
Fourth time, selling a low quality crypto portfolio of 10 names or so in this year's January. Paid basic fee, set time to 3 weeks again. Sold them for $65, and booked a loss again.
So I would say, approach Flippa with caution, don't try to sell questionable quality hand regs there (or anywhere else in general), and don't pay for upgrades if you don't have a capital that can sustain a $240 loss. Hopefully you guys can learn from my mistakes.