In 1975, MITS advertised the new Altair (not actually the first personal computer, but close enough that many historical accounts proclaim it as such) by mail order, and it sold very well by the standards of the time.
In 1984, Michael Dell started selling PC clones by mail order from his college dorm room, and founded a company that remains today one of the big names in the business.
In the nine years between, I have found essentially no mention of anyone selling computers by mail order. There is much discussion of dealer networks e.g. Paul Terrell's Byte Shop (the first Apple dealer) and Tandy (which acted as its own dealer network with several thousand stores, helping to make the TRS-80 for some years the world's bestselling personal computer), and some of department stores e.g. Commodore's bold and surprisingly difficult move in getting Kmart to sell the Vic-20, but always brick and mortar retail. As far as the historical accounts I have read are concerned, the question of mail order sales simply does not arise.
Was anyone selling a significant number of computers by mail order? If not, why not?
Edit: Raffzahn makes the excellent point that there were indeed plenty of mail order ads in the magazines of the time by small hardware companies, in many cases for computers now long forgotten. This hadn't really registered with me before, so let me rephrase my question:
During those nine years, as far as I know, there were no mail order sales of bestselling computers. The pattern was always that niche computers were sold by mail order, bestsellers by brick and mortar.
One reason for this pattern, in one direction, was that brick and mortar dealers wanted to be selling the bestsellers, not the niche machines. Okay, that's logical enough.
Why did none of the big players supplement brick and mortar with mail-order sales? Were the extra sales (even at substantially increased profit margin) not enough to be worth alienating the dealers?
Why did nobody between MITS and Dell make it big purely in mail-order sales? Conjecture: nothing changed after 1975; it was always the case that mail order would only get you a few thousand sales because mainstream customers were not comfortable buying expensive products that way; the big change came in the mid-eighties when the IBM PC clone industry started commoditizing computers enough that mainstream customers started feeling assured they knew in advance what they would be getting. Does that make sense?