eCommerce upscaling problem

Hello fellow BH'rs
I'm running a couple oscommerce shops (my day job) and 40 WH & BH websites with aff. crap like eGay, CB (not making sh1t anymore), Amazon, Ads3ns3 etc...

Now is my problem that i can't get my ecommerce to a higher level. I have multiple domains with shops for each niche and a master shop to "RULE THEM ALL". I get my most sales from Adwords and some advertising sites in my country (Netherlands) (eBay look a like's). I think more products is not always better (when i started with 100 products i made the same when i got 750 products). Maybe i will see more sales with 7.500 or 75.000 products but i don't have them

All 7 shops are selling, but not fast enough. Is it a good idea to keep building webshops (50, 100, 150, 500?). I will end up with a sh1t load of webshops and not all of them are making sales. Do i need to keep the shop even if i get 1 sale in 1 or 2 weeks (profit $3 - $10 per sale).

Maybe someone can tell me how to see this. Maybe i think the wrong way and just have to keep building and buy small amounts of new stock (max. 5 products from each).

An example is iPhone cases..... I sell +- 50 different cases, pouches, sleeves. Just 5 or 8 of them are selling, the others once in a few weeks (some are not selling at all). Do i need to upscale the cases with 100 or more cases? Or do i need to sell (get rid off) that sh1tty selling cases below wholesale prices?

Big Thanks

Edit #1: I have tried to pay more per click with Adw0rds but i will go broke if i go higher. I did that and sales getting up (not weird if i get more traffic i sell more) but i made some money day 1 day 2 i loose and so on at the end only big G is laughing .......

Maybe you should concentrate on one store that has the most potential for margin and use SEO to build up its ranks. Make unique content for the descriptions and allow for customers to write reviews for the products.

Do you make customers sign up for an account when checking out? Sometimes having too many products confuses customers. Can you try to do some split testing on the websites with different landing pages and with/without account checkouts?

Once that is done, then you can adjust the inventory I guess. What's your conversion rate? An average can be anywhere from 1-5%.

One last case scenario would be to sell a website or two to pay for SEO.

you need to concentrate on 1 site.
once you are happy with the income move to the other site and so on