Be sure to price out your options. You will find that Yahoo! and 2CO will pay off only up to a certain point, then they become exorbitantly expensive for what they do. IOW, they are valuable if you are a low-volume merchant, but will bleed you dry if you become a moderate-volume merchant.
I actually sat down and mapped it out in an Excel spreadsheet, to figure out at what point I would break even with a merchant account. The result was that I went with the merchant account much sooner than I might have otherwise.
I don't know the reasons for being with Yahoo! Small Business, maybe they have unique benefits that make them very valuable to you. Personally, I won't buy anything through a Yahoo! merchant, because my payment info is none of Yahoo's business.

I don't like the concept of "Big Brother" overshadowing the transaction. But that's just a personal hang-up I have, I might just be weird that way. (Not the first time I've been accused of being weird, LOL)
You can pick up a very affordable merchant account at ModernAuthorize,
Merchant Accounts - Credit Card Processing - ModernAuthorize. I'd love to sell you my own merchant account services

but they are a bit overpriced and I know from community feedback at the Modernbill forums that ModernAuthorize's service is good and it works. I also know from shopping around that their rates are excellent.
I do
not recomment 2CheckOut. The reasons are many, and it goes back through a long, long history. The quick version:
- The incidence of double-charges to the customer's credit card is fairly high -- higher than it should be.
- When they correct a double-charge, sometimes the funds are subtracted from the merchant's account, when they shouldn't be! The net result is 2CO keeping the single charge's funds and the merchant getting none of it. Sad, but true. I have a hosting client who is a 2CO merchant who's had this happen to him twice now... and he's not exactly a high-volume merchant. (Maybe 30 transactions a month, total)
- If a client does not recognize the charge or does not remember what the charge is for, 2CO will not even attempt to fight the chargeback, they will just let it go through and you lose the money from what was a legitimate sale.
Truth be told, I am no fan of
any third-party payment processor. The payment processor simply has too much power and can screw you out of your legitimately earned money far too easily. They all do make mistakes, and some of them (including 2CO and PaySystems/Revecom) are complete boneheads about support. As in, there is no support with a functioning brain.
(I'm sorry, these are harsh words, but nobody's going to be helped here if I don't paint the picture as clearly as possible, from my experience)
You get several key benefits from having your own merchant account. They are:
(1) Transactions are in your business name. This looks professional and it looks like you are a "real" business, not some fly-by-night. (Now if you are not a fly-by-night and you use a 3PP, don't get all excited. The trouble here is simply association -- that there
are fly-by-nights which also use 3PPs. That's why the 3rd Party Processor industry is a tough one to survive in... it's
very high-risk.)
(2) You don't have to worry (as much) about a chargeback due to the client not recognizing the charge. Hopefully they know your name or the name of your business.
(3) If you are a higher-volume (over 20 charges/month) or higher-dollar (over $1500/month, maybe less) business, the fees are lower on a merchant account.
(4) You have
100% control over what exactly is run through your merchant account. You also have
100% control over returns/refunds. You collect all of the customer's data and you are in control of it, you don't have some other basically unknown 3rd party storing it who-knows-where and doing possibly God-knows-what with it.
(5) The common merchant account gateways (like Authorize.net) work with any number of e-commerce scripts. Like, literally plug-and-play.
I too started out with a 3rd Party Processor, I used Revecom (Paysystems) who double-charged one of my clients 2 months in a row, and then charged back his "correction" to
my account, to the tune of over $100. They ended up pocketing the money, I never saw it. Not once, but
twice. Needless to say, I was steamed!!! Unfortunately they were based in Canada so there was nothing I could do, nobody I could complain to. I ended up just posting my experience over at
Web Hosting Talk in the hopes that someone else might be saved similar trouble.
Issuing refunds was also difficult with Revecom. Finally I got near that point where a merchant account was only slightly more expensive in terms of monthly fees, and I made the jump -- and have never looked back! It is also extremely handy that as business has grown over the years, the merchant account has been able to handle it with no extra work and no issues whatsoever. It scales perfectly with the business.
We do accept Paypal for payment, strictly as a matter of convenience, because Paypal is so prevalent. As best I can tell, our transactions have been handled pretty well. So as long as they keep the fees within the ballpark, and keep the service on the up-and-up, we'll stick with them.
Hope this helps, somehow!!!

Bailey