If this is the case, would the original seller financing have to mention that the buyer (me) can re-sell the property at any time? 

If you foreclose and take it back then you can resell it later but if you are owed $25,000 and it sells at foreclosure sale for $40,000 you don't get paid $40k.

These will also have different mature/resell dates. 

The other business I run is reselling, listing items on eBay & FB.

Lenders stopped allowing rent by the room due to modifications done to the property resulting in functional obsolescence and significant cost to cure in the event there was a default and they needed to foreclose on the home and resell it, given that the main buyer pool for single-family homes are primary occupants. 

However I may have been able to resell it as is and still make close to $100k without any work.  

If they won't let you build on it, explore doing a lot tie agreement so that you can combine the parcels and when you go to resell it, you can ask for a higher price point based on the large lot.

I'm a licensed real estate agent and my family rehab's homes and resell them for profit.

We wouldn’t want to resell within a year for the reason you mentioned, but we’re starting to think it’s a smarter strategy to buy and quickly fix up a duplex or more to first get our foot in the door.

And they execute their investment strategy/model (develop, resell, hold, etc.) effectively to generate a return and/or equity and make a living.