Do you guys think that I can find software app dev client here on BHW?

I am a developer and I can build desktop / android / ios apps, just wonder is it possible to create a sale thread to sale my service,

Sure, if you can develop relevant apps or software, there are people here who might be interested. Some guys are already selling their bots and tools here.

Being the biggest IM forum in the World, I suppose you may find a ton of potential buyers for your service(s).

Yes, Start with the Hope!

Yes, you can create a sales thread for this service.
Find out more about the process here -
https://www.blackhatworld.com/seo/the-black-hat-world-marketplace-rules.1295700/

You can also offer it under beta testing forum if you want to gauge the quality of your service.
Find out more about the process here -
https://www.blackhatworld.com/seo/service-reviews-beta-testers-section-rules-procedures.1270545/

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