If you want to sell items on your website then you really have a basic choice if you want to get your customers to buy from you. You could purchase SSL certificates so that their personal data is secure or you can use a third party payment processor that has their own.
There are pros and cons to using both of these. With an SSL certificate you will have to pay for it. This could be quite a high cost, that you may or may not be able to afford. However, it does give you control of the payments and means that you will not have to send your customers to another website in order to make their purchase.
You will have to pay for a merchant account and these can have various fees. There may be a cost per transaction, a percentage and a yearly fee, but this will vary between the different processors. A payment processor will also have fees. They may charge per transaction, take a percentage or other fees as well. You can find Comodo SSL certs and WildCard SSL certificates at this page: https://www.gogetssl.com/wildcard-ssl-certificates/.
It is therefore important to consider the costs of both options. However, you can of course, put up the prices of the items that you are selling to cover these costs. It is not always this easy though as you need to stay in line with your competitors and not be just too expensive for your customers to afford.
However, as well as cost you need to consider your customers and their needs. They may prefer to go to a trusted third party payment processor than to use your merchant account. They may not trust your site as they may not have heard of it and therefore prefer to use the payment processor. They may not like the payment processor and prefer to make payments directly. It is very difficult to know what they might like and therefore keep everyone happy.