A table and some rules I use for my 5e setting, inspired largely by the tinkers in the Kingkiller Chronicles, as well as something I saw on the excellent Signs in the Wilderness blog.
Tinkers
“A gold piece, a silver piece, and a copper piece”
“A tinker’s advice pays kindness twice”
A Tinker’s Debt Is Always Paid
Once for any simple trade. A character who trades with a tinker will generally find some use for the items they received, often more than they expected. (Tinkers have a knack for offering things PCs will need for some upcoming challenge in the adventure).
Twice for freely given aid. Aid in return, such as tips or directions; additionally, all rolls against the character have disadvantage for a while (DM’s judgement).
Thrice for any insult made. All rolls against the character have advantage for a while (DM’s judgement, dependent on the severity of such insult; generally, I wouldn’t recommend more than one encounter.).
Also, if a tinker offers a bargain that the character doesn’t accept, always find a reason for them to need the thing they could have gotten.
Roll up a random Tinker:
d6 Appearance
1 Pack almost as big as the bearer
2 Pushing a small cart
3-6 Riding a (1-3) shabby cart/(4-6) brightly painted cart, pulled by a (1-3) eland/(4-6) elk/(7) yak/(8) ox/(9-10) pack dog/(11-12) Plainsbird.
d4 Attitude
1 Cheerful
2 Friendly
3 Alert
4 Sleepy
d100 Advertises 9 of:
1-2 Silk scarves
3-4 Writing paper
5-6 Sweetmeats
7-8 Belt leather
9-10 Black pepper
11-12 Fine lace
13-14 Bright feather
15-16 Small clothes
17-18 Rose water
19-20 Heath blankets
21-22 Feather-plumes
23-24 Plains-melons
25-26 Pipe-leaf
27-28 Summerwine
29-30 Moose cheese
31-32 Rice cakes
33-34 Goat cheese
35-36 Sharp knives
37-38 Sweet apples
39-40 Juicy peaches
41-42 Pure salt
43-44 Stead Ivory
45-46 Wild ginger
47-48 Warm furs
49-50 Tea leaves
51-52 Sour Lemons
53-54 Sweet Plums
55-56 Sugarcane
57-58 Apricots
59-60 Pomegranites
61-62 Olives
63-64 Paadria hats
65-66 Straw caps
67-68 Rice wine
69-70 Sharp arrows
71-72 Bells
73-74 Candles
75-76 Iron Chain
77-78 White Chalk
79-80 Healer’s kit
81-82 Holy symbol
83-84 Ink and inkpen
85-86 Lamp and lantern
87-88 Soap
89-90 Tinderbox
91-92 Whetstone
93-94 Artisan’s tools
95-96 Gaming set
97-98 Musical Instrument
99-100 Herbalism kit
d12 Service/Skill
1-6 Knife-sharpener
7-8 Cobbler
9-10 Story-teller
11-12 Knows trails and back ways for miles
d4 Looking For
1 Supplies – rope, cloth, food, tea
2 Something to pass the time – alcohol, pipe-leaf, a musical instrument.
3 Something to use as a gift – a silver locket, shell beads, a golden bracelet, fine gems.
4 Tack and saddle
d4 In Need Of (but does not mention):
1 His feet are bare and bloody.
2 His cloak is more patch than fabric, and full of holes.
3 His stomach growls audibly.
4 Wearing the remnants of a hat.