We as toy train enthusiasts have essentially "moved on" from those MPC/LTI days when Lionel and its dealer network played games with SKU's that were under-produced. In addition to F3 B-units, you can add vista dome (usually regular size -- not full-length) passenger cars and diner cars (i.e., SP Daylight and N&W Powhattan Arrow) that were also part of the dealer Christmas Stocking Stuffer packs that often garnered premium price-tags.
VADarthDad's recollection is accurate indeed, in that those 18103's were going for $500 VERY shortly after they hit the street. A classic case of supply-and-demand -- although somewhat artificially manipulated. Back in the day, dealers often held on to their stock for a few weeks (i.e., claiming they were sold-out on phone inquiries)... knowing very well that folks would start getting anxious after hearing too many "sorry, they're gone" responses. Then a few weeks later those dealers would magically "get another shipment from Lionel" (LOL!), but the price would be -- yep, you guessed it -- $500.
Not the most consumer-friendly of times, for sure. And the fact that now most enthusiasts can't even give that stuff away from that era (much less attempt to get any premium dollars for them)... should be a lesson to us all that if things seem/smell/feel amiss, they often are.
The collector mentality may still have its following of sorts today, but I think most of us have grown wise to the antics that are often too common-place when items are tagged as RARE. I can't even remember when I last saw a dealer ad in the magazines that had a price listed as "CALL". And back in those days, that was a VERY common practice since the prices on those "highly sought-after collectibles" were so volatile. Today, I'd wager the number of folks who would might pick up the phone to call a dealer if they saw that in an ad would be far and few between. A few dealers tried that with Lionel's VL Big Boy... and look what happened... they still have them for sale.
David