Another thing to consider with purchasing smaller contracts. It's best to buy them at the same resort with the same use year, otherwise you have multiple contracts that you have to manage for banking, borrowing, using the points within the appropriate time frame.
We have four contracts (from Disney) with the same use year. Three different resorts (also can cause problems since you don't get the 11 month advantage for a single resort with three different resorts). We have a single contract that we bought from a reseller (small 50 point contract). It's the same resort as two of our other contracts, but a different use year. We have to carefully track those points so they don't get lost.
The more use years and resorts you put into the mix, the more confusing it can be. When you buy from Disney, they will only sell you the same use year. Unless you purchase a totally new 150 point contract.
Plus if you buy resale, the contract retains the contract number of the original owner. I don't think you get a new number (unless maybe they had multiple contracts from Disney). When you get the same use year from Disney, they just add a suffix to the original contract number. For example, your first one is 12345.0; the next would be 12345.1 and so on.