I could do this with deafault, varibles and cashmod.
Onactivate:Client:Centerprint:<color:FFFFFF>Would you like to buy (Enter name here.) for (Money amount you want to sell it for)?
Onactivate:Self:Iflastmsg:Yes
ifvaribletrue:Self:Spawn item:(Item you want.)
ifvaribletrue:Self:deductcash:(Money amount you wanted to sell it for.)
That is about it.