I wouldn't count on turning a profit over and above the savings from ordering two kits. People seem to list assembled Bottlehead amps on Head-Fi for about $100-150 over list price, but they don't sell too quickly, and my guess is that most of those actually sell for roughly the cost of a new kit + shipping. There are a number of reasons for this, but the three primary reasons are:
1. Most people can't get past the headline price of a kit direct from Bottlehead. If a kit costs $500 from Bottlehead, they are reluctant to pay more than $500 from someone else, even if it is fully assembled. Sure, they know that the labor is valuable, but the psychological hurdle in order for them to actually pay for it is often too much to overcome. Moreover, people on Head-Fi are used to paying LESS than list price for gear sold through the For Sale forum, not more. Of course it is an apples-to-oranges comparison (used commercially built amp with inflated list price, vs. essentially new, hand-built amp with low list price), but there it is.
2. The lemon problem. Unless you regularly build amps professionally, nobody has any reason to trust your soldering and assembly skills. Note that this applies even if your "day job" involves soldering electronics. You are essentially an anonymous, unknown quantity. You could be the greatest electronics whiz who ever lived, or you could be a novice, rube, hack like me.
Even if you claim the amp is working perfectly, how can a buyer know that there isn't a cold solder joint that will become defective as soon as you ship it out the door?
3. Shipping. Even if you are a soldering ninja, you might be a lazy, careless or ignorant shipper. I bought a fully-assembled Smack on Head-Fi. I have no reason to believe the seller was in any way dishonest, or that he did anything less than a perfect job building the amp. But it arrived in roughly a million pieces because he didn't pack it properly and failed to account for the physical stress caused by shipping. The same thing happened to me with a used Mapletree Audio Ear+ that I bought, so I have basically sworn off used tube amps -- they are far too delicate.
The multi-unit savings are substantial in their own right, so I'm not trying to dissuade you from buying two amps, I'm just trying to make sure you don't have unrealistic expectations for what you can sell the 2nd one for.
If you do build two Cracks, I think it would be worthwhile to hold onto them for a little while and run a couple of A/B comparisons with different upgrade components to see what delivers the biggest improvement (after the Speedball).
Best,
Adam
