You should be able to 5box up to 25, but I imagine it'll take a while. My guild hit level 12 last night and it's just me. That's probably about 5 months to get to that point, but I slack a lot so I rarely cap daily guild XP.

As far as selling a guild, as long as it's in game currency, it's fine. There's posts all over the battle.net forums where people are buying and selling guilds for gold. But it's an unsupported transaction, so if you get scammed, don't hold your breath on a GM being much help.