That they actually know the ins and outs, not just the ins, of the products they sell.

I had an expensive redo of a job when I ran out of a certain product and the supplier's sales rep turned up and handed me a bottle of something like I requested, but the bottle looked a bit different, and had a mildly different label with one word being not the same.
Naturally suspicious, I asked about it, to be told " That's just a new label, it's still the same product!"
NOT!
Chemically it was significantly different and the banner I used it on exhibited horrible cissing all over.
After contacting the manifacturer, I was told that my suspicions were correct and the product had changed quite a bit.l
I of course had to swallow the mess and make a new banner, buit it was an ordeal I'd rather not have done.
Similarly I was offered a new product on the basis that "It's exactly the same as..." by the hapless sales rep. a couple of years earlier - yet the new product, despite all precautions, failed to adhere to the substrate. My loss in redoing three signs, I suppose, because I am a mug for trusting what the sales rep said...
Loyalty is about lots of things, but product knowledge in what is will NOT do, (its limitations) is as important as the blurb about its features.
I come from a corner of the industry and a corner of the country also where signs are expected to 'last forever' metaphorically speaking, and the long term durability or longevity of many more modern digital products is simply dreadful - again, encouraging my suspicions when told how good some new product is, by the rep.
I'm loyal to those from whom I've had the fewest disasters... but supplier product changes without proper knowledge or reason put me on guard. (Traditional Signs, Ian) 

Relationships with your suppliers is as important as those you have with your clients. What I look for in my suppliers is integrity, value for money, trust, honesty, reliability, willingness to assist and an organisation that stand by their product. At the end of the day price is a factor however it is not the be all and end all (Steve Baker Signs - Steve Baker)

Quality Products, Fast delivery and Good stock levels

The products these days are all good for the purposes that your correctly using them for.The most important issue is the continuing service to the clients and naturally the price of the goods so we can have that competitive edge. 

Who I buy from depends on a number of factors.

(A) The actual products they sell must be brand names I am familiar with.
(B) Price is important but not to the extent that I would buy a shonky product.
(C) The supplier must be reliable.
(D) The supplier won't get my business if they think I'm an idiot who will pay some outrageous price for a product that I can get elsewhere for a fraction of the price they are asking. (I does happen)
(D) I won't cop it if the supplier continually ups the price and doesn't think I will notice.
(E) It certainly does help if I like the rep. There are those that I haven't "clicked" with and I'm reluctant to buy from them.
Through the years being able to get along with the rep and feel we have a "personal" relationship has been a big factor in where I buy from.

Product knowledge, industry knowledge, easy-going, minimal pressure (no matter how hard you try, we aren't going to buy 10 rolls of some product just because you have it on sale), understanding of service levels and respect for time. Quite a list - no wonder nearly everyone gets it wrong! And yes, these are definitely "enough" to keep us loyal provided that pricing isn't ridiculous. Brand and products are important, but a good rep is a huge asset to both the customer and the company they are with.

Good pricing, good delivery times, good accounting procedures and knowledgable reps. Support also when it is required is vital for capital equipment.