Hi Micah and welcome to the forum,
The problem with shipping large items is always a major one, but there is no inexpensive way around it, unless the item is in some way flat-packed for reassemble. Of course not just size, but weight is also an issue with shipping.
OEM Merc products would come from Merc themselves and I seriously doubt you would get any access to those without being a part of their dealer network.
If you're just kicking off, then doing the sweep out around the house is a great way to cut your teeth in the market, it can also be a great way to get an idea of other products to sell, by judging how well your products sell. I don't think anyone should ever be pushed over the edge to be really honest, not in this business. It's super important that you don't jump until you know you are 100% ready because it can be a seriously hard grind finding space for yourself and unless you are 100% prepared for the bumpy ride, you can tend to get very disillusioned very quickly and end up giving up before you've really given it the best effort to succeed.
So my advice, if any doubts at all, don't push yourself or let anyone push you. Take the time to strengthen the areas you feel best need to be strengthened and then you will be much better prepared for the ride.
On a side note, I do like your thinking about the Merc parts. Probably not going to happen, but it shows you're really thinking about it and that can only benefit you long term.