The Market is where you buy the categories of items not sold by the Merchant's Guild. The market owners have a deal with the Merchant's Guild - guild members can buy items at lower cost, though you never need to worry about this fact. During the night time slot the market is closed, so unless there are items on sale which you cannot wait until the next morning for, such as price down items that may revert from the morning announcements, it is better to wait as the Evening time slot has more customers than the Night time slot. This way you will be also able to buy goods both at the Market and from the Merchant's Guild during the same time slice. The only functions of the market are to buy and sell.
The Market is where items not sold in the Merchant's Guild can be found. This includes equippable items, restorative items and decorative items for your store.
As your Merchant Level rises, Flooring, Wallpapers, Venders, Counters and Carpets will also be available. Unlike the Merchant's Guild, the Market is very inconsistant with how item Tier upgrades effect what it sells as none of its categories have 6 items total, amounting to 1 per tier. Initally you can only buy Tier 0 and 1 items with a max stock of 3-5 and 1 respectively. Upon reaching Merchant Level 4, Tier 2 becomes accessible and the general trend is that the previous tiers now have infinite stock, and new items are introduced with a maximum of 1. Certain exceptions to this rule exist:
Magnet Necklace, Old Cape, Fleece Muffler, Golden Pedastal and Golden Scales become infinite without a new item appearing in the next tier.
Vacation of Eldon, Heretical Floor, Checkered Wallpaper, Heretical Table and Heretical Carpet unlock at Tier 4, but do not become infinite at Tier 5.
Books, Medicines and Decorative items may be sold out periodically without you purchasing them, though only if they normally stock a single item.
Thief's Ring can have up to 3 available, rather than just 1.
Food items in limited supply will often have up to 10 available.