Several things you might try to use as "negotiating tactics";
1.) As mentioned above, CASH (or a check) innately carries a 2%-4% premium over a credit card, which he surely accepts (all retail locations do these days).
2.) A bore light AND a bore scope inspection will show pits in there (or dust which will look like corrosion) which can be shown to him during your inspection.
3.) Do you need other supplies that he carries (powder, caps, round balls, round ball mould, gun case, suppository gun ammunition)? Offer to buy those too from him which will increase his overall transaction profit.
4.) See if you can find recent transactions on more "off the beaten track" web sites you can cite at a transaction price < $300. A print out of those can be helpful.
5.) If you can, see if you can engage him in a conversation about the merits of "inventory turnover rate" and "revenue per square foot" of retail space. Even if he doesn't do those calculations they are intuitive to anyone that does retail transactions. Odds are about 100% that he has other inventory categories that turn over much much faster.
6.) If you can figure out his cost basis (often it's around 40% on used guns vs. around 10%-15% on new stuff) you might be able to get inside that margin. You won't get the whole 40% (my guess is his cost basis is around $175-$200), but you might get down to half of it.
7.) Soothe his ego and remind him what a good customer you have been for a lot of years.
8.) Show up at what is generally a very slow time of day, month, and year. Saturday mornings right after the first of the month (right after people get paid) are NOT the time to walk in the door. Likewise, right before or after the 1st of the year (when Christmas credit card bills are coming due) is a good time and generally slow for retailers. The down side to that is he may be feeling pretty flush with cash after the Christmas season, and less willing to negotiate.
9.) You might come in to the shop and just drop a comment while just browsing that "that under hammer is still here huh? Hmmm." After a while (particularly if you're the only guy that seems to ever look at it) it will start to gnaw at him and he will get sick of continuing to look at it on his shelf.
10.) Do you have a product or service that he needs or could be used as "currency" to trade with him?
You are playing a game here. Never be afraid to walk away from a transaction.
You could lose a small gem for the sake of 25 or 50 bucks!
