It is obvious you are unfamiliar with current prices, the dynamics of how those prices work, and the nuances of high end vintage fountain pens. You speak like if all Waterman 52 and Waterman 55 pens in eBay are basic commodities with a determined price. That's not the case. There are a lot of variances for those pens/nibs in terms of exterior color, trim, clips, country of manufacture, a huge array of nib options, not to mention the actual condition of those pens, how smooth the nibs are, how they write, who serviced them, etc. Each one of those variances affects the value of those pens.
Just because you scored a low price in one or more Waterman 55 pens in eBay does not imply that is the ongoing price for all Waterman 55 pens. In general, the Waterman 55 pens that sell in eBay for little are those that have significant issues, worn or incorrect parts, etc. And since sourcing correct parts for those pens is a fairly complicated endeavor, folks who know a thing or two about these pens normally pass on them. The uniformed buy them, thinking they are scoring a bargain, and since they do not know a whole lot about those pens, they cannot even conceive why they got it for such a bargain price. They do not know that, in most cases, they bought a pen with significant flaws!
Also for your reference, even the parts in a Waterman 55 pen have a high price as many parts in Waterman 55 red ripple pens are interchangeable with the Waterman 7 red ripple pen, an iconic and highly collectable pen, that if fitted with a Pink nib fetches at least $1k in eBay in unrestored condition and coming from a seller who knows zero about those pens and who cannot guarantee all parts are correct in those pens. Likewise, just the section part alone for a Waterman 55 cardinal red can sell for $150+, its correct feed for $75+. A correct gold plated lever box for a Waterman 55 pen sells for $150+ and it will cost you about another $40-$50 plus two shipping fees to get it professionally installed onto the pen body ... and that is if you know what the correct parts for those pens and are, and are lucky to be able to find one for sale.
Some of us have been doing this for a while longer and participate in pen shows, are heavily involved within the fountain pen community, and keep track of prices and trends with a more holistic approach than merely comparing prices from eBay pens from questionable provenance, pens who may be in questionable condition, and perhaps with incorrect or worn parts, etc.
Vintage Waterman pens are considered some of the best vintage fountain pens. They were so good at making fountain pens and nibs that they became the largest fountain pen manufacturer in the world for several decades at the end of the 19th century and early 20th century. Since you are looking at flexible nibs, it is important to mention that the Waterman Pen Company perhaps produced the best vintage 14k flexible nibs ever for fountain pens. These two criteria alone makes any oversized vintage Waterman pen with a fairly flexible nib a very valuable and a very desirable pen in the current dynamics of the fountain pen marketplace. You are getting into the cream of the crop, the best of the best, in terms of vintage fountain pens with vintage 14k flexible nibs, and the prices for these rarer specimens fetch high prices, and much higher prices if they have been professionally restored, have correct parts that are not worn or patched, have flexible nibs that have been professionally installed, and come from a seller who knows and stands behind what he or she sells.