Revealing mistake: While Barney's explaining to Andy his new interest in realty they both walk to the back room, and just as they pass through the doorway the camera tracks too far to the left, so we can see the edge of the set wall and the rest of the back wall is missing. (The following shot is from the direction of that missing wall).
Character mistake: Barney claims he would be losing $3478 on the real estate deal not going through. Considering the houses only were selling for that amount or less there is no way he was making that amount of money on 5% commission.
Suggested correction: I have no idea where you got the idea the houses were being sold for less than $4K. In the beginning Barney says Andy could get $24,000 for his house and $27,500 for the Williams' house (which is also how he got the figure that Andy can get into the Williams' house for $3,500, that's not the price of the house). Those 2 houses alone are $51,500 and 5% would be $2575, and he was losing out on 2 more houses.
That has bothered me forever! I thought the house Andy was thinking of buying was 3500 not 35000! Now the amount of commission makes sense although 35000 is LOT for a house in North Carolina in the early 60's! Thank you for clearing that up.
Just watched that episode and yes, you are correct on the math. But my problem is, new houses in this era were around $8000 - $8500 new, so these older houses would have been worth nowhere near $24-$27k.