my main concern would be the advertising costs. what do you think?
Advertising costs are the tax you pay for having an unimaginative service with no competitive advantage against established, entrenched competition. So the cost will be high. Count on little, if any, word-of-mouth.
Who wants a clone, with no detectable difference (and sorry, just because you believe there's a difference obligates nobody in the market to agree with you). Excepting, of course, the one big difference: You'll have little to no volume of traffic to motivate either a client to post on yet one more board, nor a worker to bother looking.
Bottom feeder sites for penny ante chislers are a dime a dozen. Everybody wants to do one. Find some way to upend the mechanics of price over judgement sites, you'll have uncovered a competitive advantage. Price is no substitute for judgement. Lowest bids are not the road to the best value for clients.
Viewing what's being bid on as a lowest price commodity won't foster long term high value clients for the bidder.
All these sites do is oversupply workers and undersupply work. The mechanics of the site demand a plunge to the bottom, price-only competition which often takes short cuts, that doesn't support value for the client. So you see the CraigsClones giving way to the 99Clones, which in turn give way to the FiverrClones.
Five bucks?! I am waiting for the next logical step of the bidders paying the clients. My guess is it will be started by the same colleges who came up with the concept a college education so cripples your ability to work for pay you have be an unpaid intern to relearn how to work.
The reason these sites exist is price is a nice, comfortable data string for programming to use. Price is easy. Value is hard. Value is like talking rainbows and unicorns to a programmer -- if it does exist out there, somewhere, value ain't the programmer's problem. (As the programmer sees it)
Unfortunately a site based on value competition would be the competitive advantage you'd want. Because practically no programmer would conceptualize how you'd do it. Not that there aren't ways. Just that the concept is alien to everybody in the market space right now.
Sort of like the MP3 player or smart phone market, pre Apple. The market is viable -- but not for a copycat.