How do you increase the amount you can put up for sale bud? Didn't know that was something that can be done!!!
Sell, sell, sell.
You gain market rep based on every 100k roubles of items you sell. Once you hit rep 10, you get 4 slots, rep 30 gives you 5 slots. It goes all the way up to rep 1000 for 100 slots.
Currently at 27.4, I think 5 slots will be plenty though as items sell quick.
You'll also make considerably more money on most items selling via flea market as opposed to traders with the exception of a small selection of items.