Understand that you wish to increase sales.
And it certainly appears that you have the background and skills to produce/provide quality systems.
But there are two immediate concerns:
1) How much additional capability do you have to build more systems - enough to double or triple current sales? Time, space?
2) Financially, can you really continue to manage the expenses and cash flow with respect to those higher sales?
Basically, any middle-man is going to cost you. Either via commission or just a straight up per month expense whether they sell anything or not. Commission expenses are high. Local consignment businesses around here are now asking for 40% of the selling price.
And you will lose control of the "ordering" process. Middle-man organizations are likely to make any promises necessary to make a sale - delivery dates, quantities, warranties. Especially if there are some wholesale requirements.
And there are predatory organizations that do go around telling small businesses they will "sell" for them. Probably some up-front cost to you plus some monthly "retainer". And they lock you in for as much time as possible. Be wary and beware!
I suggest doing some analysis regarding the demographics of your area. Who else is selling computers: big box, small PC retail businesses, free lancers such as yourself.
Then ask who is buying?
Would expect at this time that you have saturated the "friends and family" market, and probably the related "referrals" markets. I.e., happy customers (gaming folks) telling other people. Perhaps even a few businesses or organization that you have sold to but just do not need more systems. Possibly you have been out posting flyers with tear off telephone numbers - see quite a few of those on bulletin boards in coffee shops, some public bulletin boards, have found them crammed under my windshield wipers.
Unfortunately it takes quite a bit of work to expand sales and that will take away from your time to actually build systems. Very common situation for many small business folks. Proverbial "rock and a hard place".
Honestly my sense of things is that you will need to expand into other services - e.g., networking support. However, any expansion poses other risks and problems which can be addressed at a later time.
With the trend to low cost devices that can simply be plugged into a TV HDMI port then bluetooth connected to a keyboard/mouse to estabish a computer there is a slowing demand for desktops. Simple, cheap, portable..... Desktops are not dead but the future is not good for a lot of growth with respect to the general population. Does depend on where you live to some extent.
Summary: No middle-man organizations. I really doubt that any of them could really help with sales at costs (tangible and intangible) that you can afford. Or be willing to pay.
Instead take a more quantifiable look at your area, your competition, and where you might be able to cull out a few more sales as you currently do business. But look for a niche area, that does not seem to be well addressed and serviced. You may be able to expand into that niche.
Tough to direct you away, in a sense, from your passions and dreams but most small business people, who survive as a business have to do that. Often more than once.