An excellent and well documented summary of some of the flaws and limitations of eBay's search in general, and the keyword substitution portion of the search expansion feature in particular.
Keyword substitution seems to be based on matching search terms to purchases that are made, so if users are searching for one thing and wind up purchasing something else, spurious correlations can sometimes wind up being made by the software.
Keyword substitution is an automatic feature of search expansion, like implicit category navigation that changes the category based on the keywords used.
https://www.ebay.ie/pages/help/search/expanding.html
Both of those features can be disabled by using Advanced Search Operators -- parentheses and commas (this,that) to search for this or that; quotes around individual words or phrases to search for the literal keyword only; minus signs to exclude certain terms like -octopus.
https://www.ebay.ie/pages/help/search/search-commands.html
For the benefit of anyone else who is running into similar problems with their searches, I would suggest using workarounds to deal with the limitations of those search features.
So for example #1, I would suggest the following search to bypass the spurious substitutions:
256gb "msata"
This ensures that all of the results actually contain the literal "msata" term.
For example #2, I would suggest the following search:
"optiplex 320"
That will match titles that have the word optiplex immediately followed by the number 320, and not include those titles where there is an intervening word between those terms.
Examples #3 and #4 are due to the way that the search treats a number keyword as a match to the same number with any alpha character appended, unless those combinations are explicitly excluded, as the original poster has done.
For example #5, the following search:
"Gigabyte" 1150 Motherboard
should eliminate the spurious GB substitution, as the OP has suggested. In cases where a model number contains 1150, a more specific exclusion may need to be used to eliminate those results, such as -sdh1150. Or filters can be used to specify cpu package type.
Clearly eBay's search, and the search expansion feature in particular, is not perfect.
eBay is interested in maximizing the amount of sales, so eBay has an incentive to help users find what they are looking for with a minimum of difficulty. The user at the margin is probably less interested in finding an exhaustive list of every possible search result and more interested in finding the "best" quick match using imperfectly chosen intuitive search terms, without having to learn search syntax rules.
It makes a certain amount of sense that eBay would prioritize those marginal intuitive users -- after all, that is where the money is. Power users that want complete, logical results are left to discover search syntax rules and workarounds to bypass the intuitive "close enough" search features.
eBay could do a better job of making those trade-offs explicit. Perhaps an actual switch to disable the search expansion features would be a good idea, in addition to updating the help pages so that the search rules are actually made clear.
