Start before the scale is on. Dump the lot onto a clean tray and sort by karat stamp first, broken or unmarked pieces second. Every piece gets looked at under a loupe, every clasp gets read, every chain gets checked end to end because karat stamps migrate to findings that do not match the body of the chain. A 14k clasp on a 10k rope is common. Weigh them as one unit and you pay 14k money for 10k metal, or you pay 10k money and the seller catches it on their own scale at home and never comes back.
Sort into clear deli cups, one karat per cup, plus a cup for testing. Anything stamped but suspicious goes to the testing cup, not the karat cup. Anything unstamped goes to testing. Plated, filled, and bonded pieces come out of the lot entirely and go into a separate tray you will hand back. Do this part out loud. The seller watches you sort, hears you say the karat as each cup fills, and the buy becomes a conversation about what is in the lot rather than an argument about the final number.
Pennyweight versus gram, pick one and stay there
Most counter scales read both. Pick the unit your payout sheet uses and lock the scale to that unit for the entire transaction. Switching mid-lot is how math errors happen. A pennyweight is 1.555 grams, and if you weigh three cups in grams and the fourth in dwt because someone bumped the unit button, the fourth cup pays roughly 1.5x what it should or roughly 0.65x what it should depending on which direction the error runs. Neither outcome is good. Tape over the unit button if your scale has one that toggles easily, or buy a scale that requires a menu dive to change units.
Calibration is a habit, not an event. Run a check weight across the scale at open, after lunch, and any time the scale gets moved or bumped. A 100g check weight costs less than one bad lot. If the scale reads 99.94 or 100.06, recalibrate before the next buy. Drift is normal, ignoring drift is not. Keep the calibration log on a clipboard next to the scale, dated and initialed, because when a seller disputes a weight three weeks later you want a paper trail showing the scale was checked the morning of their buy.
Tare matters more than people admit. Weigh the empty deli cup, write the tare on the cup in marker, and subtract every time. A 4.2g cup weighed as part of a 31.5g 14k reading turns into 35.7g of payout if you forget. That is a $200 mistake on a single cup at current spot. Better practice: zero the scale with the cup on it before adding gold. The scale does the subtraction for you and the seller sees the display read zero before any metal hits the pan.
Document the lot so the seller sees what you see
Every cup gets weighed in front of the seller, weight written on a buy ticket in a column next to the karat, and the seller initials each line as it is recorded. Not at the end, line by line. This sounds slow. It is faster than relitigating a number after the cash is on the counter. The buy ticket has four columns minimum: karat, gross weight, unit (g or dwt), and pure-gold equivalent at the karat ratio. The math from gross to pure happens on a calculator the seller can see, not in your head.
Photograph the lot before and after sorting, with the buy ticket visible in the after shot. The photos take ten seconds and they end disputes before they start. Keep them tied to the ticket number for at least ninety days. If your state requires a hold period, the photos satisfy part of the inventory documentation requirement and save you from pulling the actual lot back out of the safe to verify a complaint.
Pay from the pure-gold total, not the gross. Your payout percentage is whatever you have set against spot, applied to the pure equivalent of each karat group, summed at the bottom. Show the seller the sum, show them the percentage, hand them the cash. A clean ticket beats a friendly closing line every time, and the sellers who appreciate the clean ticket are the ones who come back with the next lot and tell their cousin to come too.
The counter discipline is the whole game. Sort before you weigh, lock your unit, calibrate on a schedule, tare honestly, document line by line. None of this is clever. All of it is the difference between a buy desk that compounds and one that bleeds.