Just so you know, this doesn't really make a difference. Let's say I sent my bitcoin to someone who lived in a country with capital gains tax, and they sold them -- they wouldn't need to pay capital gains tax either, because they haven't made any capital gains.
So really, the service you are offering is pretty generic money laundering (you obfuscate the source of someones money, so they can get away without paying tax). Which I won't judge on it's ethics, but you should be aware it puts both you and your customers at massive risk (with penalties that frequently involve prison).
But besides, I also think this is a pretty foolish way to do tax evasion. Let's say I have a small amount of capital gains tax I want to brazenly avoid paying, I think it'd be a lot better to just find someone on localbitcoin and do an OTC trade. If the tax office ever found out and believed you didn't do it intentionally, you would probably just have to pay back taxes and penalties. But if someone used a service that is marketed explicitly for tax-evasion, then ... ouch.
And for larger amounts of money, it would be crazy to take all that risk to just avoid tax. If it's pure capital gains you're talking about, seems like it would be pretty straight forward and legal to just relocate to a country without such tax.
Welp. That changes things. Looks like this whole thread is illegal! I'm shutting down operations with current clients and am locking this thread, thanks RHavar.