We have been doing this for over 30 years, now. Customs can be profitable, but are generally very hard to sell, for a number of reasons. Repair/flip is the easier method for sales. However, it must be done 100% properly. If you are just patching a car and kicking it out the door, then expect issues when an angry new owner brings it back to you. When we do them, the entire car is triaged, and major assemblies are disassembled, and the interior generally comes apart. Lots of cleaning in places you didn't know existed, including undercar, engine bay, etc, will expose new exciting problems with the car you did not know about. Better for you to find out than the next guy. Be prepared to spend 2x whatever you think it will cost to properly fix the car. There's always more wrong than you think. Also, plan on spending 4x as much time as you initially think. This will get you closer to the reality of it.

Another thing. Pick a type of car, and stick with it. Previous to rescuing Crossfires, I exclusively flipped Dodge SRT-4's. Fun cars, good market, hard to find well running ones. I would fix them to near perfection. I gained a reputation for how clean my cars were, and people would fly in from across the country to buy one from me. I had spares and all the proper special tools, and most importantly, the knowledge, as I was dedicated to that one platform. Before them, there were other platforms, but I always specialized in a certain type/manufacturer. This kept the spare parts assortment more reasonable and organized, and kept making it easier for me to manage the projects. I never had one come back to me because of anything I did wrong. It pays to be knowledgeable.

When listing the car, you can use terms like, all maintenance completely up to date (if that is in fact true), or recently overhauled, etc. I have found it counter-productive to list everything done to the car. Buyers do not usually think like us 'car guys', and instead think something must be wrong with the car, or that it is a lemon. I will usually hit major high points like, new tires, or new paint, and skip the details. I will add a line that complete listing of recent history available to prospective buyers. I will then generate a document that has all of the parts or details taken care of. I have had someone ask me for that something like twice.

I just sold @QuadPrism 's 04 coupe this past weekend. I used this exact strategy. It sold right at my target price (which was a fair price for what it was), and despite having a line in the ad that complete service information available, the couple who purchased it did not ask for it. That doesn't make them bad buyers. The car, its accurately described condition, and how it was presented in person made for a lack of need for it. Trust was established. You have to treat your buyer with a lot of respect. If you respect them, their time, and their future with the car, they will feel both comfortable with you, and are more likely to leave with the car happy. In order to be trustworthy, the car needs to be as you described. I will often point out all of the flaws to a prospective buyer. I would much rather them come and see the car, and be impressed with it being better than I described, than the other way around. Trust. I do not like sending a car down the road with skeletons in the closet. It just doesn't work out for anyone that way.

So this trust will usually have a cost, and that cost is your margin. I will wind up shrinking my margin as I am going along, by fixing things I can tell are not an issue now, but they will be in the not so distant future. Take the car above. My son had been driving it for the last couple of years, without major problems, but when I went to inspect the car to sell, I found that the ball joints were worn, and the steering linkage had a little play in it. It wasn't bad, and it wasn't even noticeable when driving - but I knew it was an issue waiting to happen. Parts were on the order of $250, plus a half day's worth of labor, and then a $100 alignment. $350, labor, plus bringing it for the alignment and the time spent there. It was done. It chewed $350 out of my bottom line, but I felt good about selling the car, and my integrity will remain intact. It goes back to that respect, and the knowledge that steering and tire wear are probably not going to be things the new owner is going to be upset about, and who knows? Maybe they will enjoy the car enough to get involved in our little community. I hope they will.
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