(14:49:21) <`21> !alch magic longbow
(14:49:21) -[IM]RuneScript- *** [ ALCH ]: [M] Magic longbow | High Alch: 1,536gp | Low Alch: 1,024gp
(14:49:25) <`21> !ge magic longbow
(14:49:25) -[IM]RuneScript- *** [ GE ]: Results: 2 | [M] Magic Longbow: 1,371gp 1,443gp 1,515gp (Today: -2gp) | [M] Magic Longbow (u): 1,174gp 1,236
(14:49:28) <`21> !Ge nature rune
(14:49:28) -[IM]RuneScript- *** [ GE ]: Nature Rune: 224gp 236gp 248gp (Today: No +/-)
(14:49:54) <`21> !calc 1443+236
(14:49:55) -[IM]RuneScript- *** [ CALC ]: 1443+236 = 1,679 (1.6k)
(14:50:05) <`21> !calc 1679-1536
(14:50:05) -[IM]RuneScript- *** [ CALC ]: 1679-1536 = 143
With buying at trade price you lose 143 per alch with magic longs, so that's why.