I also enjoy building PC's but I have never had anybody outside my circle of family and friends ask me to build them a computer. You might have a few people asking for advice or tips, like what components to purchase, but there is no profit to be made.
I actually started to refuse to build for others. If I can't make 20% on the sale, it isn't worth my time (IOW, if it sells for $1k my cost is $800 -- if it sells for $2k my cost is $1600, etc.).
People will show me an ad for a *fill in the blank
* for $499.00 and ask if I can beat it. I tell 'em, "That's a VERY good deal, you should buy it!"
I hand them my card and tell them to call me if they have any problems (after their warranty runs out, of course
). I've found this to be far more profitable than having $$$ tied up in a build and warranting it for a year. A $100.00 Nuke and Pave with a 30 day warranty and NO out of pocket expences is very nice, indeed!
I've already made more $$$ in the last few days doing 'house calls' than I did on my last two builds!