The word venue derives from the Latin for to come, and implies a place that people come to. When referring to online selling a venue is a place to list your items for sale. In the truest sense a venue should provide you the space to sell your items and the rest should be up to you. The venue, itself, should no place restrictions on the way your business is to be run. In reality (virtuality?) the opposite is more often the case. Sites like eBay put extensive limits and restrictions on your freedom to run your own business, preventing you from linking to most off-eBay sites, determining the payment methods you may accept, etc. and binding you with an extensive set of rules and policies that many find contradictory, if not down right uncomprehensible. Main Street Mall Online (MSMO) is near the other end of the spectrum operating with a small, very simple, easy-to-understand, code of conduct.
Venues typically provide store fronts and/or listing formats. Some allow you to use your own HTML and/or templates. Some provide what are, in escense, stand-alone websites and you are responsible to bring all the traffic. Some are more like communities, or malls, where customers move easily from one store to another. Venues support various selling methods including auction and retail.
Charges for selling on venues may be a flat store rental, a fee per listing (often variable depending on the price of the item offered for sale), final sales commissions (with amounts or percentages varying with the final sale price), or a combination of any or all of these.
If you decide to go the venue route be sure to check carefully to understand costs, responsibilities and, generally, if the venue you choose allows you to conduct your business as you intend to do.