If you're doing a lot of flips, you will be considered a dealer since the income from an S-Corp passes through to your personal tax return. 

If so, what would it matter if the contract purchaser is a dealer? 

Maybe your perception was different, but I remember the sense we had at Wacker Neuson going through rounds of layoffs, dealers defaulting, order cancelations and no sense when the construction equipment industry would recover, or if there would even be a full recovery - ever.. nobody had ever seen anything like it. 2008 was bad, I think Lehman Brothers collapsed in October, by 2009 we had to assume the equipment industry would never be the same.

PA & OH allow 4-5 homes a year to be sold w/o a license; PA requires a vehicle dealers license for new homes, but a traditional RE license is ok for used homes.

IMO, if you have strong suspicions (especially if they are the dealer!)

If you are flipping a lot of homes you could be considered a dealer and not be eligible for any capital gains treatment.

Imagine if we buy car from used car auto dealer that still take bus daily.

One of the most knowledgeable folks on here about this is @Matthew Irish-Jones and I would strongly recommend connecting with him. 

Best thing to do is ask who you bought it from, or call local modular home dealers for referrals.

They're targeting credit card companies, banks, hotels, airlines, utility providers, food delivery service, restaurants, auto dealers, event ticketing companies, and last but not least, property management companies.