Well, I know this place is rather cluttered, but this is answered around here several times.

You have to edit the market_ships.ini... this page should help you.
http://zeus.duwx.com/flsdk/tutorials/shipmarketref.php

Note that a base can sell only THREE shipsat a time. If you add one, you have to remove another. You will need a tool to uncompress the INI files, to be found here:

http://zeus.duwx.com/flsdk/downloads.php

Sorry @ lancersreactor for linking to a foreign site, but I myself had a dead hard time finding the BINI tool around here because it's so well "hidden". =/

peace,
D_R

Added:
each section stands for a base, each line for a ship. gf6_package is the "Eagle" for example. Looking up which ship is which package is rather confusing, unfortunately. Read a bit around here to find out.

marketgood = gf6_package, 30, -1, 0, 0, 1, 1, 1

means a ship is not available. to make it available, change it to

marketgood = gf6_package, 30, -1, 1, 1, 0, 1, 1

D_R

Does that mean that you can add ship buying to a base that wouldn't normally have that function or am I asking something out of turn?

Wallace

Well, I'm not all sure - but I tried adding a new "ship vendor" to a planet that didn't have one before once and he simply didn't show up, so maybe not.

Although I must admit that Iwas just toying around back then and so I wasn't too careful while doing it, maybe it was because I hadn't added _all_ ships to the new vendor. (i only added those that _would_ be on sale in the end, not the unavailable ones)

I'd advise you to rather modify an existing vendor. There are a lot of places selling ships, and no matter where you are you should have one in reach.

mata ne,
D_R

Thanks for the links, I'll give the tools a try... BTW, since I have the server currently running with people playing, will making these mods hurt the existing universe?