I have been doing
SEO (Search Engine Optimization) and SEM (Search Engine Marketing) through PPC (Pay per Click) for many years and I would never suggest that a client concentrate only on
SEO or only just SEM or just one engine.
Unless you are in the unique position that you have almost no competition using SEM, significant time and effort is required to make a successful SEM campaign with PPC.
Sure you can buy traffic through PPC, but unless it is targeted and tracked properly and you understand the value of a customer and the ultimate cost of acquisition, the traffic you may be buying may end up only draining your bank account...then again you may find that you may be able to make 30 % or more for every dollar you spend.
EBay was (and may still be I can’t say for sure) the largest advertiser on Google. Yahoo and EBay are teaming up so expect some stiff auction site competition.
Is your budget enough? Quality research will answer that question. Research your competitors sites, PPC ads on the various engines, and heavy keyword research.
My personal opinion is Google work best for most of my clients but that does vary by industry and even KEYWORD with the same industry!
Remember, traffic is not your goal. A customer who buys (whatever a customer is in your business model) is!