I think you can never go wrong if you just tell the truth. "I start with a box mix that I add different ingredients to" works well. This day, with all the food allergies, you need to be up front with what your cakes are made out of. That aside, when you're selling cakes, you should be honest about what you're using. Besides, it's not a bad thing to use a mix.
Good luck with your decision!
Angie
I agree with the honesty approach. I would never tell someone it was homemade if it were not. I always tell people that I start with a box mix, but usually only tell them what I add if they ask.
I agree that you should be honest about it. Most boxed mixes taste so good that people don't know the difference anyway. ALL my box mix cakes taste better and moister than any I make from scratch (of course, maybe that's just my lack of baking skills, lol). For that reason, I would prefer to know when a cake is from scratch and when it is a mix.
I would say it's a started with a mix but that it's had other things added etc. I would go with the whole if they dont ask dont offer. Hope that helps
I like what Cupcake13 said! LOL
People usually don't ask me that question though. If they do I'd tell them I doctor up my good ole' Duncan Hines mixes. If they want scratch I'll do scratch but unless they ask I won't. 
There was a previous thread on this, that I just don't have time to find now...
However, the poster said that she replied that she used a commercial base (boxed cake mix) to which she added her own creative touches 
At least, that's pretty much what I remember 
HTH
I do tell people that I used a box mix, and that I add my own secret ingredient, and then add that it is NOT a box of pudding mix! Everyone always thinks that is the way to get a good moist cake when that is just not true. Everyone who has eaten my cakes loves them, and a few people when they find out it is a box mix (who did not know about my secret ingredient speil) ask if I added pudding mix to it and when I tell them no, they are surprised that the cake came out so moist without it! Just goes to show that there are many once a year cake baker people out there that have the mix, and the box, but just don't follow the directions on the back.
My $0.02
I'd say tell them the truth, if your cakes are good they won't care what you use.
I use undocotored BC mix for my cakes and I haven't sold very many but I've been honest. Last night I was at a big dinner where a cake of mine was served and someone who had bought a cake from me before said they didn't know what I did to a mix but my cakes were very good. 
The funny part to me was that this woman is a home ec teacher and she can't figure out how my cakes turn out so good.
I wouldnt lie about it. But it is still homemade, just not from scratch. I tell people I use a box mix as a base for the flavor, then I add many superior ingredients than what a boxed mix calls for.
I think you should tell the truth. If the person is asking when they are deciding if they want you to make the cake, you should definitely be honest. Some people have sensitive tastebuds and can tell the difference between scratch and box. I am that way. Also, some people might just want to know what they are paying for.
When asked if I use boxed mix my standard reply is "It depends what kind of cake you want." Truthfully, sometimes I DO bake from scratch, but, have found that a "doctored" cake mix is easier and more predictable than scratch. Plus, it's FUN "playing" with the mix to see how many different textures and flavors you can get! Besides, I watched a recent episode of "Top Chef" and they used Betty Crocker White Cake Mix for a Wedding Reception Challenge. Harold (who ended up WINNING) told the judges that he had used cake mix in the past without any complaints! So, I figure if a TOP CHEF can do it?????
When asked I tell them that the base for my cake is a boxed mix.
Debbie
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