You have the program, so the latter half of the statement is your need. May I take it that you don't have a manual? Despite that, FileMaker is largely intuitive, and easily learnt if you are prepared to play with its set-up and use without expecting to set up a system with calculation fields and lookup files at your first attempt. It should be possible to get a good grasp in two to four hours.
I use FMP6 in Tiger for just the purpose that you mentioned. The file also serves as a lookup for other databases for taxation management, built or modified Macs, general inventory and any data that can usefully be extracted. I began a catalogue (now) containing 25,000 (data) records of music on LP first, and later CD also, for music programmes on radio. FileMaker also prints the programme lists for publication. I've been using FileMaker since it was called just that, and then later versions, on Plus, SE, SE/30, Classic II, Mac II line, LC II, 9600 and CRT iMacs.
Once you have understood the record (a collection of fields) and field concept, it pays to start by defining the fields that you need and their 'types', even though you can add or remove fields later. Changeing field type is a little more limited. Just a default layout for your record structure is enough to begin. Again, you can change the layout of a record later, and have multiple layouts for different emphases within the one database. Fields are easily manipulated in size and typographic style. Records can be viewed singly or continuously (think of Adobe Reader). Sorting is a snap. Global replacements are a snap, but only of the complete contents of a field. Many mathematical calculations suitable for business operations, as well as standard arithmetic and algebraic (including Boolean) calculations, are possible.
Fields are manipulated according to familiar processes and tools (think of any *Draw program). Fields have types (text, date, number only, and so on) for sorting and calculating. Data are easily exported as delimited (comma, tab) text into other apps, or imported from delimited text. The best way for you to discover how easy and intuitive FileMaker is is to start using it. I could email you a template for eBay work, but you would learn little from it until you had pulled it apart, by which time you could have learnt as much by experimentation on your own.
Plunge in. The water's warm.
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