I've heard there is a way to increase the resale price of items sold in shops. Does anyone know where the data file is located that has this option?

Found it.

%/GOG Games/Battletech/BattleTech_Data/StreamingAssets/Data/simGameConstants/SimGameConstants.json
-> There, look for "ShopSellModifier"; default is 0.1024, change that to 9.1024 and simpy buy and resell items for massive amounts of cash. I suggest making a backup of the file beforehanded, though.

The file name is SimGameConstants.json
For the Windows version it is located in
\Program Files (x86)\GOG Galaxy\Games\BATTLETECH\BattleTech_Data\StreamingAssets\data\simGameConstants

I highly recommend using Notepad++ to open and edit it as regular Notepad may corrupt the file.
Notepad++ is a free program that you should be able to find using a search online without much trouble.

I think the value you would need to adjust is "ShopSellModifier" which is 0.1024 by default. I assume 1.0 would be full price. You could set it to perhaps 0.25 for a more generous 25% sell back of your used items. The merchant has to make some kind of profit so it should make sense as some value significantly less than 1.0.

I strongly suggest you avoid using any value higher than 1.0 because it may break the game at some point. If you just need cash you can probably add a few million but I would then change it back to some value below 1.0 before something breaks and you lose your save.

You can also adjust other values in the same file to reduce expenses and increase starting cash. I would avoid setting any expenses to zero because it is possible that could cause seomthting to break as well but you could reduce them by a large amount.