For any unidentified item, first pet test if possible (pets avoid or move RELUCTANTLY over cursed, otherwise it's UC/blessed) in absence of an altar. Then go to a General Store or item-specific store and price-check (I always price-check all items in the store as well). Bear in mind a few things though.
1) Charisma affects buying (3-5 X2, 6-7 x1.5, 8-10 X1.33, 11-15 X1, 16-17 X.75, 18 X.67, 19-25 X.5), but not selling price (always half nominal price, exception see next). Nominal prices can be found in lists on the wiki--I print these lists so they're handy) .
2) Tourists under Levl15 and others wearing a visible t-shirt get charged X1.33 (how this works in conjunction with charisma? I 'm not sure) , and get paid 1/3 instead of 1/2, nominal price regardless of charisma.
3) Blessed and cursed items often (but not always) get an unspecified surcharge both buying and selling (I'll post if I remember what it is).
4)When selling, remember to say NO if you want to keep it. Also, try selling the same item a few times, shopkeepers often try to buy things at a reduced price. Use the higher price as a rule.
5) Use #name to remember what the nominal price is if you still are.t sure (ex; c100 is "cursed $100," uc100 is "uc/blessed.$100," etc. for pet-tested/price-checked items). Using the \ key, you can check to see which other same-priced items you've already found to narrow the possibilities.
Hope this helps, and may Tyr be with you.