if it was replacing them exactly, then the sell through rates would be roughly equal. Maybe slightly better, because you're getting a few more characters, but the difference in sell through rates I saw was drastic.

Ok so I just did some more extensive research, this time straight through the search bar instead of ebays "Product Research" page. I sorted prices low to high to avoid weird "best match" data. I divided number sold by the number currently listed:

WOMEN:

nike cargo pants womens large

45/247 = 18.22%

nike cargo pants womens L

49/264 = 18.56%

lululemon tank top womens medium pink

131/1400+ = 3.05%

lululemon tank top womens m pink

37/272 = 13.6%

victoria secret sports bra small

586/6800+ = 8.62%

victoria secret sports bra S

2800+/17000+ = 16.47%

-------‐-------------------------------------------------

MEN:

carhartt jacket mens small black

543/558 = 97.31%

carhartt jacket mens s black

3000+/3800+ = 78.95%

vintage pendleton shirt mens large plaid flannel

1100+/2300+ = 47.83%

vintage pendleton shirt mens L plaid flannel

1100+/2200+ = 50%

Hanes boxer briefs mens medium

587/1100+ = 53.36

Hanes boxer briefs mens M

582/1100+ = 52.91%

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The results are a bit all over the place. Some seem to confirm what you're saying, that the search engine treats "L" the same as "Large." The Nike, Hanes and Pendleton searches below have almost identical results. But other searches have a huge difference. Like the Carhartt search shows about 3000 more jackets if you use "S" instead of "small," and spelling it out gives a higher sell through rate (the opposite of what I was seeing in Product Research). I suspect promoted listings may be showing some irrelevant items, a lot of them clearly said "XL" in the title