Autocrat,

This message is a whole 2 months old and I have not see anybody respond yet.

I am kind of doing the same that you are doing, but I work as a Data Architect for a large company in Canada and am doing an eCommerce like site's Database design.

The big difference is that this site if for internal use by employees of the company I work for. But the Product catalogue will be populated with products from external vendors. My company is just creating the site as part of a rewards and recognition system where employees can spend there points to buy items from the catalogue.

What I have done is that I have a CATALOGUE_ITEM as a super type (not sure if you understand this data modeling terminology) which is sub-typed into PRODUCT(something tangible) and SERVICE(non-tangible). I then have a child table to CATALOGUE_ITEM called CATALOGUE_ITEM_CHARACTERISTICS. This is where I will record all the "characteristics" about a CI. This is where I was having issues explaining my design to others and this is where your eCommerce site will differ.

But for your purposes, what you should build is then a table called CHARACTERISITC_TYPE (such as Color, Gender etc) and CHARACTERISTIC_TYPE_VALUE. This second table is where you will list all the valid values for a CHARACTERISTIC_TYPE. Then build an intersection table for these two called ALLOWED_CHARACTERISITC_TYPE_CHARACTERISTIC_VALUE
This will be where you define valid combinations of Type's and Value's. For instance a type of Gender does not make sense to have a value of Red, Blue, Green etc, but Male, Female...

These two tables and the intersection table is what then you will give the end user and admin screen for which they can add any values to.

The catch is that things such as Weight, Length, Width are not CHARACTERISITC. I will create them as columns on PRODUCT as they are really specs of a PRODUCT, but not really variations in which the PRODUCT is available. This concept is debatable and this is where I was having lot of discussion with my colleagues.
The best would to be look at what eBay does as they really sell both PRODUCTS and SERVICES, unlike Amazon which sells only PRODUCTS.
At the end of each PRODUCT detail page on Amazon you will Product details, which always contains things such as Dimension, Weight, Shipping special details and some kind of ASIN number.

Hope this helps and do let us know what you ended up designing your database like.