Ron, eBay has changed a lot in the market place as far as "value" goes. As far as promoting the Hobby, that is an age old question. Some of the stuff in my "collection" is just stuff and other things are treasures... and I paid for them. Some were deals and some weren't. I look at collecting vs using differently these days. I used to collect just to collect, now every engine I get is one I would not be afraid to use... I even look at the Hobby differently. In the beginning if was all out of fun... now it is more or less what I consider "Performance Art"... The whole collector market is driven by one thing... finding a "gem" for dirt cheap. It has never been anything else. The search is part of the mystique.
Jim is right about old sparkers.. The Super Cyclone, Dooling, Elf, Orwicks, McCoys, OKs.... they are still highly desired.
"Nostalgia plays an important role in valuing collectibles. Many individuals begin collecting because they are trying to buy back part of their childhood or youth. Collecting provides a means of escape from admitting that they are getting old." Harry Rinker
Everything in the collector market has it's value set by three factors, the amount of people who DESIRE said item, the number of said items that actually go on the block and number of people who want said item within that space and time when the item is sold and what they are willing to pay.
So where is a good place to find "fair prices"? I have sold and bought at hundreds of swap meets and Antiques Shows over the past 35-40 yrs. I still do estate sales... there is Craigslist... IMO, the best place to get deals on model stuff is from other modelers. I was at a show in Portland OR back in 2002 and saw a Cox fuel can with a price of $65 on it... and I began to wonder how the seller came up with the price? I made the remark, "there is no guide book for model airplane stuff" and I was approached by Peter Schiffer to write a book. At first I scoffed, then relented. 14 Months of research and writing went into my book. I didn't like the current "Value Guide" books on the market because they lacked "reverence". I wanted a book that was enjoyable all the time not just a dust collector reference book.
So my advice to you is this... How bad do you want the little K&B 15R? Here is what I do... I sell some of my lesser and easier to replace stuff to finance to harder pay for cool stuff. It's not about "profit" until you let your desire pay more than an item is actually worth... I have passed on 100s of cool things because the seller or bidding was silly... I have found stuff dirt cheap and sold it for obscene prices... I have lost money on many items... as far as using eBay as a price guide, if you buy on eBay, you are dealing within a group of some pretty knowledgeable and savvy buyers. So, it comes down to this, it's like fishing, there are only so many fish and there are only so many lines in the water at one time... eventually you will go to "river" and be the only one standing on the shore and that "fish" will be yours... I mean how many people want a 15R? Eventually, that group that wants one will get smaller and as desirability dwindles, prices drop. Collecting is a game, it is a lesson in patience, it teaches us about greed and human nature. Only you can decide what something is worth to you...
My dad taught me this... "There is more to fishing than catching fish." Modeling and collecting are the same way... it's about people, not everything that has value can be bought and not everything bought has value. My most treasured engines have names like Big Art, Big Jim, Tom Lay, Lee Woolard or even modelers long gone attached them.... most of all buy only things you will love or use and they will always be desirable to you and that is what counts... time is precious, there are a whole lot of us that "wished" we had bought something and passed on it because money got in the way... sometimes you have to throwout practicality and have fun. God knows how much money I have wasted on stupid junk... buy the 15R and stop thinking about money... and one day you will look back and know you went a different way... that's what modelers do...