SEO is great but be prepared to wait at least a year before anything volume goes through.

It will happen again so just prepare yourself. 

Here's where it gets interesting -Of the 4 potential buyers, they were all interested and I let the fourth buyer know we already had bids above $420,000 and I was fully prepared to take one of those offers.

@Chris Moyer This  is a joint-venture or at the very least a promissory note.

also, the roof doesn't look great from the pics, so be prepared to negotiate on that one as well (if the buyer's inspector knows what he's doing). nobody installs 3-tab shingles if they're looking for quality. 

If so, should I be seeking a lawyer to handle this/possibly prepare a countersuit?

Be prepared to sue for damages.

., you should let it linger for longer and then make another offer, but be prepared to be turned down. 

Or simply tax preparation?

Not trying to scare you but these things are serious for an inexperienced investor and easily take you out of the game in the beginning if you are not prepared.